Method of vacuum decarburization/refining of molten steel

ABSTRACT

A method for vacuum decarburization refining of a molten steel includes providing a vacuum tank having a one-legged, straight barrel snorkel as a lower portion of the vacuum tank. The a degree of vacuum in the vacuum tank is regulated at a high carbon concentration region to a value in a range of −35 to −20 in terms of G defined by the following equation (1): 
     
       
           G =5.96×10 −3   ×T ×ln( P/Pco )  (1), 
       
     
     wherein 
     
       
           Pco =760{10 (−13800/T+8.75) }×(% C)/(% Cr)  (2), 
       
     
     and wherein P&lt;760, T represents molten steel temperature, K, and P represents the degree of vacuum in the vacuum tank, Torr.

TECHNICAL FIELD

The present invention relates to a method and apparatus for vacuumdecarburization refining a molten steel and, more particularly, to amethod and apparatus, for refining a molten steel, that can inhibit thedeposition of a splash onto the inner wall of a vacuum tank and anoxygen lance and at the same time can prevent oxidation loss of metal inthe molten steel.

BACKGROUND ART

Conventional methods for additional decarburization refining of a moltensteel which has been once subjected to decarburization refining in anelectric furnace or a converter to provide a molten steel having acarbon concentration of not more than 0.01% by weight include: (1) a VOD(vacuum oxygen decarburization) method, typified by the one disclosed inJapanese Unexamined Patent Publication (Kokai) No. 57-43924, wherein anoxygen gas is blown onto the surface of a molten steel in a ladle whileholding the molten steel surface in vacuo; and (2) a straight barreltype snorkel method wherein an oxygen gas is blown onto the surface of amolten steel within a snorkel submerged in molten steel to carry outvacuum refining.

In the method (1), VOD, a satisfactory space cannot be ensured above themolten steel surface. This causes a splash of molten steel, scatteredduring oxygen blowing decarburization refining, to be deposited onto atop-blown lance and a cover of a vacuum vessel, adversely affecting theoperation.

The method (2), straight barrel type snorkel method, unlike the method(1), has no significant limitation on equipment, and an example of thismethod is disclosed in Japanese Unexamined Patent Publication (Kokai)No. 61-37912. The method disclosed in this publication is shown in FIG.35. Specifically, in this method for vacuum refining of molten steel, amolten steel 71 contained in a ladle 70 is sucked through a snorkel 72into a vacuum tank 73. An inert gas is blown into the molten steelwithin the snorkel 72 through under the plane of projection of thesnorkel 72 within the ladle 70, and, at the same time, an oxidizing gasis blown through a top lance 74 onto the surface of the molten steelwithin the vacuum tank 73. In this case, the inner diameter of thesnorkel 72 is determined so that the ratio of the inner diameter (D₁) ofthe snorkel 72 to the inner diameter (D₀) of the ladle 70, that is,D₁/D₀, is 0.4 to 0.8. In addition, the depth of blowing of the inert gasis determined so that the ratio of the depth (H₁) of blowing of theinert gas as measured from the surface of the molten steel to the depth(H₀) of the molten steel within the ladle 70, that is, H₁/H₀, is 0.5 to1.0. The above method for vacuum refining of molten steel aims toefficiently carry out decarburization without the deposition of themetal, slag and the like within the tank.

Japanese Unexamined Patent Publication (Kokai) No. 2-133510 proposes avacuum treatment apparatus comprising: a ladle for placing therein amolten metal; a vacuum tank having a snorkel, submerged in the moltenmetal, provided at the lower end of the vacuum tank; an evacuation pipeconnected to a vacuum source for evacuating the interior of the vacuumtank; and a shield disposed in the interior of the vacuum tank, whereinthe shield is kept at a height of 2 to 5 m above the molten steelsurface within the snorkel.

The method proposed in Japanese Unexamined Patent Publication (Kokai)No. 61-37912, however, had the following problems (i) to (iv).

(i) Conditions for decarburization refining, such as the flow rate ofthe oxygen gas blown onto the molten steel, the flow rate of the argongas for agitation, and the degree of vacuum within the vacuum tank 73,are not properly specified. This causes excessive fluctuation of themolten steel surface and splashing, leading to operation troublesattributable to deposition of the metal.

(ii) In the oxygen blowing decarburization refining ofchromium-containing molten steel, such as stainless steel, the chromiumcomponent contained in the molten steel is oxidized with the blownoxygen. A part of the chromium oxide produced by the oxidation isreduced with carbon contained in the molten steel in the course ofdescending through the molten steel. Most part of the chromium oxide,however, undergoes the convection due to the inert gas blown from belowthe molten steel and floats, without being reduced, on the surface ofthe molten steel between the snorkel and the inner wall of the ladle toform slag 75 which is then discharged from the molten steel, increasingthe loss of the chromium component.

(iii) The presence of the slag 75 containing chromium oxide causes thesurface of the molten steel present between the snorkel 72 and the innerwall of the ladle to come into contact with air and to be cooled. Thisincreases the viscosity of the molten steel surface. In addition, theslag 75, the metal or the like is deposited around the above inner wallof the ladle, making it difficult to conduct sampling of the moltensteel in the course of and at the end of the refining, or making itdifficult to move the snorkel 72 from the position of the ladle 70 atthe end of the refining, which is an obstacle to refining.

(iv) The oxygen efficiency in the decarburization, defined as the ratioof the amount of the oxygen gas contributed to the decarburization ofthe molten steel to the total amount of the oxygen gas blown onto themolten steel, is influenced by refining conditions, such as the degreeof vacuum in the vacuum tank 73, the state of agitation of the moltensteel, and the flow rate of the oxygen gas blown. These refiningconditions are not proper, making it difficult to maintain the oxygenefficiency in decarburization at a high level.

The method described in Japanese Unexamined Patent Publication (Kokai)No. 2-133510, wherein a shield is provided within a vacuum tank (asnorkel) to prevent splash of the molten steel created by oxygenblowing, thereby preventing deposition and accumulation of a metalcaused by solidification of the splash deposited onto an oxygen lance, avacuum tank, an evacuation pipe, had the following problems.

(i) When an exhaust gas is passes between shields within the vacuumtank, the molten steel splash in the exhaust gas or dust produced bysolidification of the splash is deposited and accumulated onto theshields, increasing the flow resistance of the exhaust gas, which inturn increases the pressure loss within the vacuum tank.

(ii) Since the spacing, between the shields, serving as a passage forthe exhaust gas becomes narrow, a high-power evacuation apparatus isnecessary to provide a high degree of vacuum.

(iii) When a metal or the like scattered by splashing or spitting isonce deposited and accumulated onto the passage for the exhaust gasbetween the shields, removal of the deposited and accumulated metalcannot be achieved without difficulty due to the complicated structureand requires a lot of time and labor.

In the method disclosed in Japanese Unexamined Patent Publication(Kokai) No. 61-37912, when the oxygen blowing refining is carried out ata high speed in order to increase the productivity of vacuum refining,the splashing is remarkably increased, posing the following problemswhich will be described with reference to FIG. 35.

(i) Although the creation of the splash of the molten steel 71 per secan be inhibited, dust is still contained in the exhaust gas. Therefore,the dust is gradually deposited within the evacuation duct 76particularly around its duct inlet section to form a deposit 77,clogging the passage or increasing the air-flow resistance, which lowersthe attainable level of the degree of vacuum within the vacuum tank 73.

(ii) Dust is introduced into a gas cooler 78 and damages the gas cooler.This results in suspension of equipment and increased maintenance cost.Further, a dust deposit is formed within the gas cooler 78, which causesa markedly lowered cooling efficiency.

(iii) Once a dust deposit 77 is formed within an evacuation duct 76, thedust is strongly united and must be manually removed. This increases thedust removal burden.

The method described in Japanese Unexamined Patent Publication (Kokai)No. 61-37912 is disadvantageous in that, for example, chromium oxide(Cr₃O₃) formed during oxygen blowing decarburization flows out from thesnorkel into the outside of the vacuum tank and, since Cr₂O₃ has a highmelting point, slag on the ladle is solidified, making it difficult tosample the molten steel, that is, posing a problem in the operation. Anadditional problem involved in this method is that Cr₂O₃, which has onceflowed out into the outside of the tank, does not contribute to a laterdecarburization reaction, inevitably resulting in lowered oxygenefficiency in decarburization.

RH—OB is widely known as a method for oxygen blowing decarburizationrefining in vacuo. When this method is used, for example, in thefinishing of stainless steel, aluminum is added to the molten steelbefore the oxygen blowing decarburization and combustion is carried outusing top-blown oxygen to raise the temperature of the molten steel(aluminum temperature elevation or temperature elevation by aluminum).In this case, when aluminum temperature elevation is carried out under ahigh degree of vacuum, the depth of a cavity, of the molten steel,formed by a blown oxygen jet (cavity depth) becomes large, leading to afear of bricks at the bottom of the tank being damaged by the blownoxygen jet, which makes it difficult to conduct temperature elevation byaluminum under a high degree of vacuum.

Further, the straight barrel snorkel type vacuum refining method isdisadvantageous in that, as can be seen in the process for producing anultra low carbon high chromium steel disclosed in Japanese UnexaminedPatent Publication (Kokai) No. 57-43924, there is a limitation on thedecarburization in a degassing period due to the difficulty ofmaintaining the agitating force and, as can be seen in the vacuumrefining method disclosed in Japanese Unexamined Patent Publication(Kokai) No. 2-305917, an attempt to improve the reduction rate in thedegassing period results in remarkable wear of refractories.

Furthermore, after the oxygen blowing decarburization, introduction ofaluminum as a reducing agent into the molten steel within the vacuumtank in order to recover a metal by reduction of a metal oxide, forexample, chromium oxide, causes a rise in temperature of the moltensteel by heat generated by thermit reaction, or scattering (bumping) ofthe molten steel or slag by a reduction reaction involving instantaneousevolution of CO gas, resulting in melt loss of refractories within thetank and deposition of the metal or slag, which is an obstacle to theoperation.

DISCLOSURE OF INVENTION

A general object of the present invention is to solve the above problemscreated in oxygen blowing decarburization of a molten steel by theabove-described RH—OB, VOD, or a refining method using a vacuum refiningapparatus comprising a vacuum tank having a one-legged, straight barrelsnorkel.

A more specific object of the present invention is to provide a methodfor vacuum decarburization refining of a molten steel that, even whenthe concentration of carbon in the molten steel is in a highconcentration region, can inhibit the deposition of a splash onto theinner wall of the vacuum tank, the nozzle submerged in the molten steel,and the top-blown lance, prevent loss of a metal in the molten steel,for example, loss of chromium by oxidation, and, at the same time,reduce the fixation between the snorkel and the ladle by the slag.

Another object of the present invention is to provide means that doesnot increase flow resistance of an exhaust gas in a passage, shields theupper part of the vacuum tank and the oxygen lance from radiated heatduring the vacuum decarburization refining, inhibits the entry of dustcreated by splashing of the molten steel into an evacuation system, andat the same time prevents clogging of the evacuation system with thedust.

A still another object of the present invention is to provide meansthat, during oxygen blowing decarburization in a high carbonconcentration region, can prevent a metal oxide formed during the oxygenblowing decarburization from flowing out into the outside of the tank.

A further object of the present invention is to provide a method foradding aluminum that, at the time of raising the temperature usingaluminum, can prevent the production of a metal oxide other than Al₂O₃and the deposition of a large amount of the metal.

A still further object of the present invention is to provide adegassing method that can efficiently produce an ultra low carbon steelwhile preventing the production of a metal oxide in the molten steel.

The above various objects of the present invention can be attained bythe following refining methods and apparatus.

At the outset, according to one aspect of the present invention, thereis provided a refining method wherein a molten steel, which has beendecarburized in a converter to regulate the carbon content to not morethan 1% by weight (all “%” in the following description being by weight)is charged through a vacuum tank snorkel into a vacuum tank in astraight-barrel type vacuum refining apparatus; and in the vacuum tank,decarburization refining is carried out in such a manner that the carboncontent of the molten steel is divided into a high carbon concentrationregion, which is a reaction region where the decarburization reactionrate is governed by the feed of an oxygen gas blown through a top-blownlance into the molten steel, and a low carbon concentration region whichis a reaction region where the decarburization reaction rate is governedby movement of carbon in the molten steel, the degree of vacuum withinthe vacuum tank is regulated for each carbon concentration region and,at the same time, the flow rate of the oxygen gas blown through thetop-blown lance is regulated to an optimal value (oxygen blowingconditions) for each carbon concentration region, and, in addition, theflow rate of an inert gas fed through a nozzle provided at a low portionof a ladle of the refining apparatus is also regulated for each region.

The above refining method can enhance the oxygen efficiency indecarburization and at the same time can prevent the occurrence ofsplash within the snorkel and the fixation of slag in the nozzlesubmersed portion.

Further, according to the present invention, at the time of oxygenblowing decarburization, particularly when a temperature elevation dueto oxidation of aluminum (an aluminum temperature elevation in thefollowing description being the same) is carried out, the degree ofvacuum within the vacuum tank in the aluminum temperature elevationperiod, particularly in an oxygen blowing decarburization period in aregion where the carbon concentration is not less than the criticalcarbon concentration region, is closely regulated according to thefollowing conditions. This can prevent the deposition of the metalcaused by splash or the oxidation of the metal.

Aluminum temperature elevation period: G≦−20

Oxygen blowing decarburization period: −35≦G≦−20

G=5.96×10⁻³ ×T·ln(P/P _(co))

wherein

P _(co)=760·[10^((−13800/T+8.76))]·[% C]/[% Cr];

P: less than 760;

wherein

T: molten temperature, K; and

P: degree of vacuum within the tank, Torr.

For example, when the steel comprises 0.1% of carbon and 3% of chromiumwith the balance consisting of iron and T is 1700° C., Pco is 1476 Torr.In this case, in order to regulate G to −20, P may be kept at 270 Torr.On the other hand, when the steel comprises 0.1% of carbon and 12% ofchromium with the balance consisting of iron and T is 1700° C., Pco is370 Torr. In this case, in order to regulate G to −20, P may be kept at67 Torr.

Introduction of aluminum and quick lime in an amount of 0.8 to 4.0 timesthe amount (kg) of aluminum added in the aluminum temperature elevationperiod and, in addition, introduction of a slag component, such as quicklime, in the oxygen blowing decarburization period in a high carbonconcentration region to maintain the slag thickness at 100 to 1000 mmare also effective in preventing splash and in accelerating thesoftening of slag.

Further, the regulation of the depth of immersion of the snorkel in themolten steel in the aluminum temperature elevation period and theregulation of the immersion depth of the snorkel in the molten steel inthe oxygen blowing decarburization period respectively to 200 to 400 mmand 500 to 700 mm can accelerate the reduction of a metal oxide (forexample, Cr₂O₃ in refining of stainless steel) by a reaction with carboncontained in the steel, permitting the oxygen efficiency indecarburization to be kept on a high level.

According to the present invention, after the oxygen blowingdecarburization, degassing is carried out under reduced pressure. Inthis case, an inert gas is injected from the low position of the ladleinto the molten steel, of which the carbon concentration has beenbrought to around 0.01% by the oxygen blowing decarburization, in suchan atmosphere that the degree of vacuum within the snorkel is in therange of from 10 to 100 Torr, so as to bring K value, defined by thefollowing equation, to the range of from 0.5 to 3.5, thereby agitatingthe molten steel.

K=log {S·H _(v) ·Q/P}

wherein

K: agitation intensity at the activated surface;

S: activated surface area (plume eye area), m²;

H_(v): depth of injected inert gas, m;

Q: flow rate of injected inert gas, Nl/min/ton-steel; and

P: degree of vacuum within the tank, Torr.

The degassing treatment can maintain the renewal of the interface at aactivated surface, which is a substantial gas/metal reaction interface,enabling a high-purity molten steel having an attained carbonconcentration of not more than 10 ppm to be effectively produced.

When introduction of aluminum for reduction, after the degassingtreatment, to reduce a metal oxide (for example, Cr₂O₃ in the case ofrefining of stainless steel) produced during oxygen blowing, therebyrecovering the metal, is necessary, an inert gas for agitation isinjected into the molten steel in the flow rate range of from 0.1 to 3.0Nl/min/ton-steel (in terms of flow rate per ton of molten steel to berefined; hereinafter referred to as “Nl/min/t”) in an atmosphere havinga low degree of vacuum of not more than 400 Torr, or alternatively, itis possible to employ a method wherein, immediately after the degassingtreatment, the pressure is returned to the atmospheric pressure, thevacuum tank is lifted, and, simultaneously with the lifting of the tank,aluminum for reduction is introduced into the molten steel and an inertgas for agitation is injected into the molten steel at a flow rate of0.1 to 3.0 Nl/min/t during the introduction of aluminum for reductionand at a flow rate of 5 to 10 Nl/min/t after the introduction ofaluminum for reduction. The injecting of the inert gas by the abovemethod can prevent a rapid rise in temperature of the molten steel orbumping of the molten steel and at the same time can prevent nitrogenpickup in the reduction period.

The present invention provides a vacuum decarburization refiningapparatus that can inhibit the deposition of splash (droplets) createdby splashing or bumping, or dust formed by solidification of the splashonto the inner wall of the vacuum tank and the snorkel submerged in themolten steel, which is a major problem to be solved by the invention.The vacuum decarburization refining apparatus has the followingconstruction.

At least one burner is provided on the side wall, in an upper tank, inthe vicinity of the canopy of the vacuum tank, and a space having alarger inner diameter than the inner diameter of the snorkel is providedin a lower tank in the vacuum tank. In addition, a shielding section,which has at its center a space having an inner diameter smaller thaneach tank and larger than the outer diameter of the top-blown lance, isprovided, between the lower tank and the upper tank at a position whichreceives enough radiated heat to melt the deposited metal, integrallywith the side wall of the vacuum tank.

The vacuum tank having the above construction permits the influence of ahigh temperature, around a hot spot created by the blowing of oxygenthrough the top-blown lance and the decarburization reaction, on therefractories in the side wall of the lower tank to be avoided, and atthe same time enables the metal deposited on the shielding section to bemelted by radiated heat. Further, dust, constituted by splash which hasascended to the upper tank without being deposited onto the shieldingsection and has been deposited in the vicinity of the canopy, is meltedby means of the burner, flows downward and is removed.

Further, the evacuation duct disposed between the vacuum tank and a gascooler for cooling an exhaust gas comprises an ascendingly inclinedsection inclined upward from an duct inlet provided in the upper tank ofthe vacuum tank and a descendingly inclined section inclined downwardfrom the top of the ascendingly inclined section. Therefore, splash ofthe molten steel and dust, which, together with an exhaust gas, haveentered the evacuation duct are collected in a dust pot provided belowthe descendably inclined section without being deposited within theevacuation duct.

As described above, a major object of the present invention is toincrease the oxygen efficiency in decarburization while minimizingsplash, bumping and other unfavorable phenomena created in the course ofrefining. Since, however, means is provided which, even when splashingor the like is created, can effectively avoid or remove droplets or dustderived from the splashing and the like, the degree of vacuum within thevacuum tank can be always kept on a desired level, realizing stableoperation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory view of a vacuum decarburization refiningsystem which is applied to a method for vacuum decarburization refiningof a stainless steel according to an embodiment of the presentinvention;

FIG. 2 is a diagram showing the relationship between the total weight ofchromium oxidized (chromium oxidation loss) and the amount of splashcreated in the aluminum temperature elevation period and thedecarburization refining period and the G value;

FIG. 3 is a diagram showing a change in G value in the temperatureelevation period and the decarburization refining period with respect tothe present invention in comparison with comparative examples;

FIG. 4 is a diagram showing the relationship between W_(CaO)/W_(Al) andthe oxygen efficiency in decarburization;

FIG. 5 is a diagram showing the relationship between the immersion depthof the snorkel in the aluminum temperature elevation period and theoxygen efficiency in decarburization;

FIG. 6 is a diagram showing the relationship between the immersion depthof the snorkel in the decarburization period and the oxygen efficiencyin decarburization;

FIG. 7 is a diagram showing the relationship between the flow rate of anargon gas for agitation in the aluminum temperature elevation period andthe oxygen efficiency in decarburization;

FIG. 8 is a diagram showing the relationship between the flow rate of anargon gas for agitation in the decarburization period and the oxygenefficiency in decarburization;

FIG. 9 is a typical diagram showing the relationship between theconcentration of carbon in the molten steel and the decarburization rateduring decarburization refining;

FIG. 10 is a typical diagram showing a change in immersion ratio (h/H)over time during decarburization refining;

FIG. 11 is a typical diagram showing a change in flow rate of an oxygengas over time during decarburization refining;

FIG. 12 is a typical diagram showing a change in reduction rate of theflow rate of an oxygen gas over time during decarburization refining;

FIG. 13 is a typical diagram showing a change in flow rate of an inertgas over time during decarburization refining;

FIG. 14 is a typical diagram showing a change in immersion depth (h) ofthe snorkel over time during decarburization refining;

FIG. 15 is a diagram showing the relationship between the oxygenefficiency in decarburization and the immersion ratio (h/H);

FIG. 16 is a diagram showing the relationship between the oxygenefficiency in decarburization and the flow rate of an inert gas in ahigh carbon concentration region;

FIG. 17 is a diagram showing the relationship between the oxygenefficiency in decarburization and the rate of a reduction in flow rateof an oxygen gas;

FIG. 18 is a diagram showing the relationship between K value and thedecarburization rate in the decarburization period;

FIGS. 19(A) and (B) are diagrams showing the step of reduction treatmentin finishing of a stainless steel according to one embodiment of thepresent invention (where neither deposition nor solidification of slagonto the upper part of the wall of the ladle occurs);

FIGS. 20(A), (B), and (C) are diagrams showing the step of reductiontreatment in finishing of a stainless steel according to anotherembodiment of the present invention (where deposition and solidificationof slag onto the upper part of the wall of the ladle occur);

FIG. 21 is a diagram showing the relationship between the flow rate ofan argon gas for agitation during the reducing aluminum introductionperiod and the recovery of chromium oxide;

FIG. 22 is a diagram showing the relationship between the flow rate ofan argon gas for agitation after the reducing aluminum introductionperiod and the recovery of chromium oxide;

FIG. 23 is a partially sectional view of a snorkel, for a vacuum tank,coated with slag;

FIG. 24 is a sectional side view of a vacuum decarburization refiningapparatus according to one embodiment of the present invention;

FIG. 25 is a partially sectional perspective view of the vacuumdecarburization refining apparatus shown in FIG. 24;

FIG. 26 is a cross-sectional view taken on line X—X of FIG. 24;

FIG. 27 is a sectional side view of a vacuum decarburization refiningapparatus according to another embodiment of the present invention;

FIG. 28 is a partially sectional perspective view of the vacuumdecarburization refining apparatus shown in FIG. 27;

FIG. 29 is a cross-sectional view taken on line Y—Y of FIG. 27;

FIG. 30 is a sectional plan view of an vacuum decarburization refiningapparatus provided with burners according to one embodiment of thepresent invention;

FIG. 31 is a typical diagram showing a change in surface temperature ofa canopy over time;

FIG. 32 is a partially sectional side view of a vacuum refiningapparatus according to one embodiment of the present invention;

FIG. 33 is a plan view of the vacuum refining apparatus shown in FIG.32;

FIG. 34 is a side view showing a dust pot attached to a vacuum refiningapparatus; and

FIG. 35 is a cross-sectional side view of a conventional vacuum refiningapparatus using an evacuation duct.

BEST MODE FOR CARRYING OUT THE INVENTION

The best mode for carrying out the invention will be described withreference to the accompanying drawings.

At the outset, a vacuum decarburization refining system used forcarrying out the method according to the present invention will bedescribed.

As shown in FIG. 1, a vacuum decarburization refining system 10comprises: a vacuum tank 15 comprising a cylindrical refractory; a ladle13 containing a molten steel 11; and an evacuating apparatus 16 forevacuating the interior of a vacuum tank 15.

The vacuum tank 15 comprises a lower tank and an upper tank. The lowertank constitutes a snorkel 14 submerged in the molten steel 11, while atop-blown lance 18 for blowing an oxygen gas into the molten steel 11 isliftably provided in the canopy of the upper tank.

Further, the vacuum tank 15 is provided with a lift drive 17 forvertically moving the vacuum tank 15, and a nozzle (a porous plug) 19for blowing an inert gas into the molten steel is provided at the lowposition of the ladle 13, for example, the bottom.

An oxygen gas flow rate control valve 20 for regulating the flow rate ofthe oxygen gas blown through the top-blown lance 18 is disposed on theinlet side of the top-blown lance 18, and an inert gas flow rate controlvalve 21 for regulating the flow rate of the inert gas is provided onthe inlet side of an inert gas suction nozzle. These control valves forregulating the flow rates of the oxygen and inert gases are controlledby a controller 23 and the like.

Further, a vacuum gage 22 for measuring the degree of vacuum within thevacuum tank 15 is provided at a predetermined position of the vacuumtank 15 or the evacuation system.

The vacuum decarburization refining system is constructed so that asignal corresponding to the degree of vacuum measured with the vacuumgage 22, a signal on the position of the snorkel 14 relative to theladle 13, a signal indicating the concentration of carbon in the moltensteel 11 and other signals are input into the controller 23 and,according to these input signals and an operating procedure describedlater, the controller 23 controls the evacuating apparatus 16 and thelift drive 17 so that the evacuating apparatus 16 and the lift drive 17perform respective necessary operations.

In determining the concentration of carbon in the molten steel 11, thecarbon concentration of the molten steel 11 may be directly measured, oralternatively may be determined by calculation based on the carbonconcentration before the refining and the history of a change inconcentration of a CO gas in the exhaust gas.

It is also possible to use a method wherein a change in carbonconcentration over time for each treatment step is previously determinedand the carbon concentration at a specified time is estimated based onthe data.

The ladle 13 is a nearly cylindrical vessel, for a molten steel, linedwith a refractory such as an alumina-silica refractory.

According to the method of the present invention, decarburizationrefining of a molten steel is carried out under reduced pressure usingthe above apparatus. Regarding a series of steps constituting the methodof the present invention, a decarburization refining process, as afinishing process of a stainless steel, wherein decarburization iscarried out through aluminum temperature elevation-oxygen blowingdecarburization-degassing-optional reduction with aluminum to bring acarbon concentration to a predetermined value, will be described by wayof example.

The step of aluminum temperature elevation and the subsequent step ofoxygen blowing decarburization will be first described.

A snorkel 14 provided at the lower part of the vacuum tank 15 issubmerged, for example, in a molten stainless steel 11 having a chromiumconcentration of 16% and a carbon concentration of 0.7% within the ladle13. The interior of the vacuum tank 15 is evacuated by means of anevacuating apparatus 16 to maintain the degree of vacuum, P, within thevacuum tank on a predetermined level. This permits the molten steel 11within the snorkel 14 to be sucked, causing the surface of the moltensteel to ascend through the snorkel 14, which, as shown in FIG. 1,results in a change in depth h of immersion of the snorkel 14 and depthH of the molten steel within the ladle 13.

Thereafter, aluminum (Al) is added to the vacuum tank and an oxygen jet24 is injected and blown into the molten steel 11 within the snorkel 14through the oxygen blowing lance 18 to conduct temperature elevation anddecarburization refining of the molten steel 11.

According to this embodiment, in the temperature elevation anddecarburization refining of the molten steel 11, bringing the G value,defined by the following equation (1), to not more than −20 in analuminum combustion period in an early stage (temperature elevationperiod) can inhibit excessive production of chromium oxide during theblowing of oxygen.

G=5.96×10⁻³ ×T×ln(P/P _(co))  (1)

wherein

P _(co)=760×[10^((−13800/T+8.76))]×[% C]/[% Cr];

P: less than 760;

wherein

T: molten temperature, K; and

P: degree of vacuum within the tank, Torr.

In the vacuum decarburization refining of a molten stainless steel, itis important to carry out the operation so as to ensure a preferentialdecarburization region in the Hilty equilibrium equation represented bythe following equation (2).

log ([% Cr]·Pco/[% C])=−13800/T+8.76  (2)

In refining under reduced pressure, an important operating factor in theapplication of the equation (2) is the partial pressure of CO (P_(CO))in an atmosphere represented by the degree of vacuum during operation,and the molten steel temperature (T) is a very important additionalfactor. Therefore, introduction in advance of aluminum or the likehaving higher affinity for oxygen than chromium and carbon followed byoxygen blowing to raise the molten steel temperature by utilizing theheat of oxidation is effective in inhibiting the oxidation of chromiumin the oxygen blowing decarburization period.

Since, however, the oxidation of chromium occurs also during thealuminum temperature elevation, the prevention of oxidation of chromiumduring the temperature elevation period has been an important factor forprevention of the oxidation of chromium in the whole stage of oxygenblowing, that is, for reducing the unit requirement for a reducing agentused after oxygen blowing is stopped.

For this reason, according to the present invention, in order to preventthe oxidation of chromium during temperature elevation/decarburizationrefining, the degree of vacuum in the aluminum temperature elevationperiod is kept on a high level as much as possible to burn only aluminumin this period.

More specifically, during the aluminum temperature elevation period, theoxidation of chromium is prevented during the temperature elevationperiod by regulating the degree of vacuum within the tank so as tomaintain the G value, defined by the equation (1), at a value of notmore than −20. This is because, as indicated by a solid line in FIG. 2,maintaining the G value at a value of not more than −20 reduces loss ofchromium by oxidation to accelerate the combustion of aluminum orcarbon.

In this case, preferably, aluminum for temperature elevation isintroduced in portions during temperature elevation/oxygen blowing,because introduction of aluminum all at once before the oxygen blowingfollowed by temperature elevation while oxygen blowing with aluminumdissolved in the molten steel creates such an unfavorable phenomena thataluminum in the molten steel within the vacuum tank is temporarily usedup during the temperature elevation period and, in this state, even whenthe G value is brought to not more than −20, the oxidation of chromiumoften occurs.

The distance between the surface of the molten steel sucked into thesnorkel in the oxygen blowing period and the canopy of the vacuum tank,that is, the freeboard, is preferably not less than 6 m from theviewpoint of preventing spitting in the aluminum temperature elevationperiod and preventing splash, created in the subsequent decarburizationrefining period, from reaching the canopy.

In this case, the term “temperature elevation period” refers to a periodbetween the initiation of oxygen blowing and the point of time when theoxygen blowing proceed to the accumulated amount of oxygen representedby the following equation (3).

Amount of oxygen blown in temperature elevation period (Nm³)=Amount ofaluminum added (kg)×purity of aluminum×33.6/54  (3)

In the decarburization refining period after the completion of thetemperature elevation, the G value is brought to the range of from −35to −20. As described above, when the degree of vacuum is such that the Gvalue exceeds −20, as indicated by a solid line in FIG. 2, the oxidationof chromium is promoted. On the other hand, oxygen blowingdecarburization under such a high vacuum that the G value is less than−35, as indicated by a dotted line in FIG. 2, leads to splashing,resulting in a remarkably deteriorated operation efficiency.

The G value in each of the above periods is regulated to a predeterminedvalue as follows. The degree of vacuum P is measured with the vacuumgage 22. The temperature T of the molten steel is previously providedbased on the temperature history for each carbon concentration predictedfrom the temperature before the treatment. Based on these data, the Gvalue is determined in the controller 23 according to the equation (1).The degree of vacuum P is regulated based on the results so that the Gvalue falls within the above range.

Further, according to the present invention, in order to avoid aoperation problem, attributable to an outflow of Al₂O₃, produced by thealuminum temperature elevation, into the outside of the tank, quick lime(CaO) in an amount corresponding to 0.8W_(Al) to 4.0W_(Al) (kg), whereinW_(Al) represents the amount of aluminum added at the time of thetemperature elevation (kg), is introduced.

In the method for vacuum decarburization refining according to thepresent invention, the resultant slag should be discharged into theoutside of the tank before the degassing as a later step. When Al₂O₃,produced by the aluminum temperature elevation as such, flows out intothe outside of the tank, however, the slag floating in the ladle issolidified in an early stage because Al₂O₃ per se is an oxide having avery high melting point. This makes it difficult to conduct sampling ofthe molten steel and, in addition, leads to a problem of fixing thesnorkel to the ladle.

For this reason, in order to avoid the above operation problems, CaO isadded in the above amount in the aluminum temperature elevation periodto form a calcium aluminate compound (12CaO.7Al₂O₃), a low-meltingcompound, improving the percentage liquid phase of the slag andconsequently avoiding the above operation problems.

In this case, when the amount of CaO added is less than 0.8W_(Al) (kg),the amount of calcium aluminate produced is insufficient, leading to theprecipitation of a large amount of a single phase of Al₂O₃, ahigh-melting oxide, which results in unsatisfactory melting of the slag.On the other hand, when the amount of CaO added exceeds 4.0W_(Al) (kg),the amount of calcium aluminate produced is sufficient. In this case,however, a large amount of a single phase of CaO, a high-melting oxide,is precipitated, accelerating the solidification of slag which hasflowed out. Further, the amount of slag within the snorkel isexcessively increased. In the oxygen blowing decarburization period as alater step, this inhibits the arrival of the top-blown oxygen jet at thesurface of the molten steel, resulting in lowered oxygen efficiency indecarburization.

Further, the depth of the snorkel submerged in the molten steel in thevacuum tank in the aluminum temperature elevation period is preferablyin the range of from 200 to 400 mm, from the viewpoint of suitablybringing Al₂O₃ and CaO produced by the oxygen blowing temperatureelevation into contact with each other in the molten steel within thesnorkel to accelerate the production of a calcium aluminate compound.When the immersion depth is less than 200 mm, as shown in FIG. 5, thetime of contact between Al₂O₃ and CaO in the molten steel within thesnorkel is so short that Al₂O₃ and CaO are discharged outside the systembefore the production of the calcium aluminate compound. This causes theslag on the ladle to be solidified, making it difficult to sample themolten steel and posing other problems. On the other hand, when theimmersion depth exceeds 400 mm, the residence time of the calciumaluminate compound within the snorkel becomes long, accelerating meltloss of the refractory in the submerged portion. This further creates anexcessively large amount of residual slag within the submerged portionin the later oxygen blowing decarburization period, inhibiting thearrival of the blown oxygen jet at the molten steel, which results indeteriorated oxygen efficiency in decarburization.

In the oxygen blowing decarburization period after the aluminumtemperature elevation period, in order to prevent the occurrence of alarge amount of splash while maintaining the oxygen efficiency indecarburization on a high level, preferably, the G value is brought tothe range of from −35 to −20 in a high carbon concentration region wherethe carbon concentration is the critical carbon concentration (0.1 to0.3 wt %) or more, and, at the same time, the following requirements aresatisfied.

(i) The activated surface is regulated so as to occupy not less than 10%of the total surface area of the molten steel and not less than 100% ofa surface blown by an oxygen gas jet, i.e., a projection surface of theoxygen gas jet.

(ii) In the high carbon concentration region where the carbonconcentration is not less than the critical carbon concentration, thedepth of the snorkel submerged in the molten steel is brought to therange of from 500 to 700 mm, and, at the same time, the flow rate of aninert gas for agitation injected by the low portion of the ladle ismaintained in the range of from 0.3 to 10 Nl/min/t, preferably from 0.3to 4 Nl/min/t, while blowing oxygen gas at a flow rate of 3 to 25Nm³/h/t onto the molten steel through an oxygen blowing lance providedin the canopy of the vacuum tank.

(iii) In the high carbon concentration region, quick lime or the like isadded at once or in portions to regulate the thickness of slag on thesurface of the molten steel within the snorkel to 100 to 1000 mm, interms of the thickness, in a stationary state.

(iv) In a subsequently formed low carbon concentration region where thecarbon concentration is in the range of from 0.1-0.3% by weight to 0.01%by weight, the degree of vacuum within the tank is continuously shiftedtoward a high degree of vacuum, and, at the same time, the flow rate ofthe oxygen gas is reduced to a range of 0.5 to 12.5 Nm³/h/t/min. At thesame time, the flow rate of the inert gas is brought to a range of from0.3 to 10 Nl/min/t, preferably from 5 to 10 Nl/min/t, and the immersiondepth of the snorkel is increased and/or decreased in the predeterminedrange.

It is known that, regardless of whether the oxygen blowingdecarburization refining of the molten steel is carried out underatmospheric pressure or in vacuo, metallic elements (iron, chromium andthe like) contained in a steel bath are oxidized with oxygen fed in thebath to form metal oxides (such as FeO and Cr₂O₃) and are then reducedwith carbon contained in the molten steel to permit decarburization toproceed.

In this connection, in oxygen blowing decarburization refining of achromium-containing molten steel typified by a molten stainless steel,the metal oxide is composed mainly of chromium oxide (Cr₂O₃). SinceCr₂O₃ is a high-melting oxide, the presence of Cr₂O₃ results in aremarkably lower percentage of liquid phase of the slag. In the methodfor vacuum oxygen blowing decarburization refining of a molten steelwherein the lower part of a one-legged, straight-barrel cylindricalvacuum tank is submerged in the molten steel specified in the presentinvention and the interior of the vacuum tank is then evacuated to carryout oxygen blowing decarburization refining, when Cr₂O₃ formed withinthe snorkel is discharged outside the snorkel in an early stage in sucha state that the reduction of Cr₂O₃ with carbon contained in the moltensteel is unsatisfactory, the reduction of Cr₂O₃ with carbon contained inthe molten steel does not take place because the slag on the ladle is ina stationary state. This leads to oxidation loss of a large amount ofchromium. In addition, the slag on the ladle becomes very rich in Cr₂O₃,and, even when the calcium aluminate is formed, the solidification ofthe slag on the surface of the molten steel within the ladle remarkablyproceeds. This deteriorates the workability and, for example, makes itdifficult to sample the molten steel.

For this reason, maximizing the opportunity for contact of a metal oxideproduced by oxygen blowing (in the present invention, the metal oxidewill be hereinafter described as Cr₂O₃ by taking oxygen blowingdecarburization refining of a stainless steel as an example) with carboncontained in the molten steel within the snorkel of the vacuum tank toaccelerate the reduction reaction within the snorkel is important fromthe viewpoint of preventing the oxidation loss of chromium in the oxygenblowing decarburization period and efficiently carrying out the oxygenblowing decarburization while maintaining the oxygen efficiency indecarburization on a high level.

One requirement for this according to the present invention is to formthe activated surface, in the oxygen blowing decarburization period, ina proportion of not less than 10% of the total surface area of themolten steel and not less than 100% of the surface blown by the oxygengas jet.

This is because the formation of Cr₂O₃ at the activated surface, whichis the most active reaction site on the surface of the molten steel,reduces the size of Cr₂O₃ particles to increase the area of the contactinterface of the Cr₂O₃ particles and the carbon contained in the moltensteel. When the activated surface is formed in a proportion of less than10% based on the total surface area of the molten steel, the reductionin size of the Cr₂O₃ particles per se does not proceed. In this case,the Cr₂O₃ remains as coarse particles. Therefore, without satisfactorilyreacting with carbon in the molten steel within the snorkel, Cr₂O₃ isdischarged outside the tank, posing problems of increased chromium lossand deteriorated workability. Likewise, when the activated surface isformed in a proportion of less than 100% based on the oxygen blownsurface, there arises a problem associated with coarsening of theresultant Cr₂O₃ particles.

Further, in the present invention, the carbon content of the moltensteel to be decarburization-refined has been divided into a high carbonconcentration region and a low carbon concentration region with thecritical carbon concentration as a boundary between the high carbonconcentration region and the low carbon concentration region, and, foreach region, the optimal flow rate of an oxygen gas (oxygen blowingrate), the rate of a reduction in flow rate of the oxygen gas, the flowrate of an inert gas for agitation, the degree of vacuum in the vacuumtank, the immersion depth (immersion ratio) of the snorkel and the likehave been investigated.

As shown in FIG. 9, the oxygen blowing decarburization refining reactionis generally divided into a high carbon concentration region, which is areaction region where the decarburization rate (−d[C]/dt) is governed bythe feed rate of the oxygen gas (a region governed by the feed ofoxygen), and a low carbon concentration region which is a reactionregion where the decarburization rate is governed by the moving speed ofcarbon in the molten steel (a region governed by the movement of carbonin the steel).

In the oxygen blowing decarburization refining of the molten stainlesssteel in vacuo, the critical carbon concentration ([% C]*), at which theregion changes from the region governed by the feed of oxygen to theregion governed by the movement of carbon in the steel, is approximatelyin the range of from 0.1 to 0.3% by weight, although the critical carbonconcentration somewhat varies depending upon the chromium content andthe operating conditions.

In the present invention, the flow rate of the oxygen gas in the highcarbon concentration region is limited to 3 to 25 Nm³/h/t. The reasonfor this is as follows. When the flow rate of the oxygen gas in the highcarbon concentration region is less than 3 Nm²/h/t, the decarburizationrate of the molten steel is likely to fall, making it necessary toprolong the refining time, which lowers the productivity.

On the other hand, when the flow rate of the oxygen gas exceeds 25Nm³/h/t, the rate of CO gas generated in the decarburization reaction isexcessively increased, leading to the occurrence of a large amount ofsplash. This unfavorably develops an adverse effect, such as loweredyield, and increased chromium loss attributable to the fact that theproduction rate of the metal oxide is excessive relative to the feed ofcarbon, in the molten steel, which should serve as a reducing material,into the snorkel.

When the flow rate of the inert gas for agitation in the high carbonconcentration region is less than 0.3 Nl/min/t, the circulation betweenthe molten steel within the snorkel and the molten steel in the ladle isdeteriorated, resulting in lowered mixing efficiency, which lowers theoxygen efficiency in decarburization and increases the chromium loss.

On the other hand, when the flow rate of the inert gas for agitationexceeds 10 Nl/min/t, a problem associated with the outflow in an earlystage of the metal oxide produced within the snorkel into the outside ofthe tank, or remarkable acceleration of the damage to refractoriesconstituting the snorkel unfavorably occurs. In this case, the upperlimit of the flow rate of the inert gas for agitation is preferably 4.0Nl/min/t.

When the oxygen blowing decarburization refining is carried out invacuo, the occurrence of splash in the high carbon concentration regionbecomes the most serious problem in stabilizing the operation. The highcarbon concentration region is the so-called “most activedecarburization period.” During this period, the evolution of the CO gasis most active, which induces splashing. Therefore, in order to preventsplashing and to carry out oxygen blowing decarburization refiningwithout causing significant deposition of the metal, the prevention ofsplashing in the high carbon concentration region is very important.

According to the present invention, in the oxygen blowingdecarburization period in the high carbon concentration region, quicklime or the like is added all at once or in portions to the tank, andoxygen blowing decarburization is carried out in such a state that slaghaving a thickness of 100 to 1000 mm in terms of the thickness in astationary state is held on the surface of the molten steel within thesnorkel.

Splashing created in the oxygen blowing decarburization is known to becreated by rebounding of a top-blown jet and by bursting of CO gasbubbles, produced within the molten steel (bubble breaking) on thesurface of the molten steel. The attainable height of the splash isgoverned by the initial speed at the time of formation of splash(initial speed) and the CO gas evolution rate (that is, flow rate of theexhaust gas). Therefore, lowering the oxygen blowing speed per se iseffective in reducing the attainable eight of the splash. The loweringin oxygen blowing speed leads directly to lowered throughput speed.Therefore, this means cannot be useful means from the viewpoint ofmaintaining the high productivity. Thus, the reduction in initial speedimmediately after the formation of splash is important from theviewpoint of reducing the attainable height and scattering distance ofsplash while maintaining the high productivity.

Further, in the present invention, in order to reduce the initial speedimmediately after the formation of splash, a suitable slag layer isformed on the surface of the molten steel. When splash particlespenetrate the slag layer, the slag layer reduces the energy of thesplash particles, thereby significantly relaxing the later scatteringbehavior.

In this case, the thickness of the slag layer to be held on the moltensteel within the vacuum tank is preferably 100 to 1000 mm in terms ofthe thickness in a stationary state on the surface of the molten steelwithin the snorkel. When the thickness of the slag layer is less than100 mm, the energy loss of the splash is small, making it impossible torelax the later scattering behavior. On the other hand, when thethickness exceeds 1000 mm, the arrival of the top-blown oxygen jet ontothe surface of the molten steel per se is inhibited, resulting inlowered oxygen efficiency in decarburization.

The composition of the slag to be accumulated on the surface of themolten steel can be provided by incorporating a slag material, such asquick lime, all at once or in portions into the vacuum tank in the highcarbon concentration region, where splash particles are most activelyproduced in the oxygen blowing decarburization period and the carbonconcentration is the critical carbon concentration or more. In thiscase, the composition is preferably such that (% CaO)/(% SiO₂)=1.0 to4.0, (% Al₂O₃)=5 to 30%, and (Cr₂O₃)≦40%. This composition can protectthe refractories constituting the snorkel and can prevent thesolidification of the cover slag. When the slag, for covering thesplash, within the vacuum tank is solidified, the effect of preventingthe splashing attained by the slag is remarkably reduced. Further, inthis case, as described above, the solidification of the slag in theladle in an early stage at the time of outflow into the outside of thetank is accelerated. Specifically, when (% CaO)/(% SiO₂) is less than1.0, the effect of preventing splashing can be attained. In this case,however, the melt loss of the refractories is significant. On the otherhand, when (% CaO)/(% SiO₂) exceeds 4, even though the otherconstituents of the slag fall within the above respective ranges, theslag is solidified. This leads to the disappearance of the effect ofcovering the splash, resulting in deposition of a large amount of themetal. Likewise, when the concentration of (% Al₂O₃) is less than 5%, alarge amount of the splash is unfavorably created due to thesolidification of the slag. On the other hand, when the concentrationexceeds 30%, the melt loss of the refractories is significant. Further,in the production of a stainless steel or the like by the melt process,a concentration of Cr₂O₃ in the slag exceeding 40% is unfavorable fromthe viewpoint of the solidification of slag.

The oxygen blowing conditions according to the present invention arecharacterized by the rate of reduction in flow rate of the oxygen gas(oxygen blowing rate) in the low carbon concentration region. In theprior art, the reduction rate in this region has not been fully takeninto consideration. According to the present invention, as shown in FIG.17, bringing the reduction rate to the range of from 0.5 to 12.5Nm³/h/t/min has realized very effective operation.

When the reduction rate of the flow rate of the oxygen gas in the lowcarbon concentration region is less than 0.5 Nm³/h/t/min, the reductionin evolution of CO gas is so small that the amount of splash created isexcessive. Further, the amount of chromium oxidized attributable toexcessive feed of the oxygen gas is increased.

On the other hand, when the reduction rate exceeds 12.5 Nm³/h/t/min, theoxygen efficiency in decarburization in the low carbon concentrationregion is lowered. Further, in this case, the excessively rapidreduction in flow rate of the oxygen gas requires prolongation of thetime of oxygen blowing at a low flow rate. As a result,disadvantageously, the productivity is likely to fall.

In the low carbon concentration region, since the evolution rate of theCO gas is gradually lowered, the occurrence of splash per se is reduced,posing no significant problem associated with the stabilization of theoperation. Further, as described above, since the decarburizationreaction in the low carbon concentration region is in a “region governedby the movement of carbon in the steel,” the mass transfer of the carbonin the molten steel should be accelerated beyond the mass transfer inthe high carbon concentration region in order to maintain the oxygenefficiency in decarburization at a high level. Further, in order toefficiently carry out degassing as a later step, the cover slag, withinthe snorkel, used for the prevention of splash in the high carbonconcentration region, should be discharged outside the tank as much aspossible during the oxygen blowing decarburization period in the lowcarbon concentration region.

In the present invention, in addition to a continuous fall in the flowrate of the oxygen gas, the flow rate of the inert gas for agitation isbrought to a range of from 0.3 to 10 Nl/min/t, preferably 5 to 10Nl/min/t, in the low carbon concentration region, and the immersiondepth of the snorkel is increased and/or decreased in a predeterminedrange.

This is done from the viewpoint of more actively feeding carboncontained in the molten steel to the metal oxide (Cr₂O₃) produced byoxygen blowing to more effectively carry out the decarburizationreaction and, in addition, of accelerating the discharge of slag. Whenthe flow rate of the inert gas for agitation in the low carbonconcentration region is less than 0.3 Nl/min/t, the following problemsarise. Specifically, in this case, the agitating force isunsatisfactory, resulting in unsatisfactory feed of carbon to Cr₂O₃produced within the tank, which in turn results in lowered oxygenefficiency in decarburization and increased chromium loss. Further, inthe above case, disadvantageously, the discharge of the slag isunsatisfactory, leading to lowered reaction efficiency in the later stepof degassing.

On the other hand, when the inert gas is fed at a flow rate exceeding 10Nl/min/t, the effect of feeding carbon into the tank is not improved.This unfavorably renders an attack by the gas more severe, acceleratingthe damage to refractories constituting the snorkel.

Even when the composition of the slag in the aluminum temperatureelevation period and the high carbon concentration region is regulated,the slag, which, with the elapse of the blowing time is dischargedoutside the tank and floated on the ladle, is partially cooled andsolidified upon contact with the air.

This in some cases causes the snorkel to be partially fixed to theladle. In the present invention, in order to avoid this unfavorablephenomenon, the immersion depth of the snorkel in the low carbonconcentration region is decreased and/or increased in a predeterminedrange. This fluctuates the surface of the molten steel in the ladle andaccelerates the heat transfer from the molten steel to the slag on theladle, causing remelting of the slag, which facilitates sampling of themolten steel and, in addition, enables fixation between the snorkel andthe ladle to be fully avoided. The variation in the immersion depth ofthe snorkel may be semi-continuously carried out in a range of from 0.1to 0.6 in terms of h/H wherein h represents the immersion depth of thesnorkel and H represents the depth of the molten steel within the ladle.Preferably, however, the immersion depth of the snorkel is varied onlyby decreasing the immersion depth from the viewpoint of promoting thecirculation of the molten steel and discharging the slag in an earlierstage. In this case, when the h/H value is less than 0.1, the dischargeof the slag is significantly promoted. This, however, causes Cr₂O₃produced by oxygen blowing to be simultaneously discharged outside thetank before the reduction of Cr₂O₃ with carbon contained in the moltensteel, leading to increased chromium loss. On the other hand, when theh/H value exceeds 0.6, the circulation between the molten steel withinthe snorkel and the molten steel within the ladle becomesunsatisfactory. This unfavorably results in increased chromium loss anddeteriorated discharge of the slag.

Next, the vacuum decarburization refining method will be described inmore detail based on the above various conditions with reference to FIG.1 and FIGS. 10 to 14.

In the high carbon concentration region, the decarburization refining iscarried out in such a manner that an oxygen gas flow rate control valve20, an inert gas flow rate control valve 21, a lift drive 17, and anevacuating apparatus 16 are controlled to maintain the oxygen gas flowrate (Q) at 3 to 25 Nm³/h/t, the inert gas flow rate (N) at 0.3 to 4.0Nl/min/t, and the immersion ratio (h/H) at 0.1 to 0.6 as shownrespectively in FIGS. 11, 13, and 10 through the operation of thecontroller 23 or by the operations of an operator while monitoring orestimating a change in the concentration of carbon in the molten steel11 within the snorkel 14 in the vacuum tank.

In the subsequent low carbon concentration region, the decarburizationrefining is continued in such a manner that, as shown in FIGS. 10 to 14,the oxygen gas flow rate (Q) is reduced at a reduction rate (R) of 0.5to 12.5 Nm³/h/t/min by regulating the oxygen gas flow rate control valve20 and, in addition, as shown in FIG. 16, the immersion depth (h) of thesnorkel in the molten steel 11 is reduced in a predetermined range byoperating the lift drive 17.

The reduction rate of the oxygen gas flow rate (Q) is the magnitude ofthe slope of the oxygen gas flow rate (Q) over the time, that is, thederivative time of the oxygen gas flow rate (Q), and is expressed inNm³/h/t/min.

Thus, according to this embodiment, in the decarburization refiningoperation of chromium-containing molten steel 11, the oxygen gas flowrate (Q), the inert gas flow rate (N), the degree of vacuum (P)(regulation based on G value), the immersion ratio (h/H), the immersiondepth (h) of the snorkel in the molten steel 11, the thickness of theslag having a regulated composition and the like are regulated torespective predetermined values, thereby simultaneously satisfying thefollowing objects (i) to (iii).

(i) Prevention of splashing also in the high carbon concentration regionwhile maintaining the oxygen efficiency in decarburization on a highlevel.

The object can be attained by maintaining the oxygen gas flow rate, theinert gas flow rate, the degree of vacuum, and the thickness of slag inrespective proper ranges.

(ii) Prevention of chromium loss.

Chromium loss occurs because the chromium component, contained in themolten steel 11, oxidized on the molten steel surface within the snorkel14 is discharged through the lower end of the snorkel 14 into theoutside of the tank and floats between the wall of the snorkel 14 andthe inner wall of the ladle 13. Therefore, maintaining the immersiondepth, the inert gas flow rate, the oxidizing gas flow rate and the likebalanced in a predetermined range permits the state of convection of thechromium component (chromium oxide) in the molten steel 11 within thesnorkel 14 to be properly maintained. This causes chromium oxide to beefficiently reduced with carbon in the steel within the snorkel 14,preventing migration of the chromium component into the slag 12.

(iii) The fixation between the outer wall of the snorkel 14 and theinner wall of the ladle 13 through the slag 12 can be avoided.

Since the relative position of the snorkel 14 and the ladle 13 is variedin a predetermined range in the low carbon concentration region, thefixation through the slag 12 can be prevented.

The molten steel, which has been subjected to oxygen blowingdecarburization in this way, is then degassed under a high degree ofvacuum.

At the outset, degassing will be explained. For both common steel andstainless steel, in preparing high-purity steels, such as ultra lowcarbon steel, by the melt process, degassing under a high degree ofvacuum should be carried out after oxygen blowing decarburization as thestep of secondary refining. In this case, it is known thatdecarburization proceeds through a reaction of oxygen and carboncontained in the steel represented by the equation (4).

C+O→CO  (4)

Therefore, maintaining the concentration of oxygen in the steel on ahigh level during degassing is effective in efficiently accelerating thedecarburization reaction in the degassing period. In particular, in anearly stage of the degassing, spontaneous evolution of a CO gas from theinterior of the molten steel (internal decarburization) is known to be amajor decarburization reaction site. Thus, maintaining the concentrationof oxygen in the steel on a high level is useful particularly in anearly stage of the degassing.

In this connection, it should be noted that, in the production of ahigh-purity stainless steel by the melt process, degassing is carriedout after decarburization conducted by blowing oxygen gas in vacuo inthe step of secondary refining. Therefore, it is important thatsatisfactory dissolved oxygen concentration be maintained by optimizingthe carbon concentration and the degree of vacuum when decarburizationconducted by blowing oxygen gas ends.

In the oxygen blowing decarburization refining under reduced pressurefollowed by ending the oxygen blowing (after a stop of the blowing) anddegassing under a high degree of vacuum, preferably, the oxygen 5blowing decarburization is carried out to [% C]=0.01 to 0.1%, the degreeof vacuum within the tank during the stop of oxygen blowing is broughtto 10 to 100 Torr, and the attained degree of vacuum in the subsequentdegassing is brought to a high value of not less than 5 Torr. Thisenables degassing refining of a chromium steel, such as a stainlesssteel, to be effectively carried out. This method is based on theoptimization of the concentration of oxygen in the steel specified byequilibrium condition of the partial pressure of CO (P_(CO)) representedby the carbon concentration and the degree of vacuum within the tank andmakes it possible to maintain the degassing rate on a high level duringdegassing.

When the carbon concentration [% C] during the stop of oxygen blowing isless than 0.01%, the oxidation of chromium during oxygen blowing issignificant due to a shortage of carbon, even though the degree ofvacuum within the tank during the stop of oxygen blowing is in a properrange (that is, 10 to 100 Torr), posing a problem that the unitrequirement of the reducing agent for the reduction treatment isincreased. On the other hand, when the carbon concentration [% C] duringthe stop of oxygen blowing exceeds 0.1%, the degassing time should beprolonged, leading to a problem of productivity.

When the degree of vacuum within the tank is higher than 10 Torr, thesolubility of carbon in the steel based on the equilibrium conditionspecified in this case is unsatisfactory, even though the carbonconcentration during the stop of oxygen blowing is in the range of from0.01 to 0.1%. The amount of oxygen to be consumed by the degassingreaction is insufficient, disadvantageously making it difficult toproduce a high-purity steel by the melt process. On the other hand, whenthe degree of vacuum within the tank is lower than 100 Torr, chromium isexcessively oxidized in the last stage of the oxygen blowing period.

The attained degree of vacuum at the time of degassing should be as highas not less than 5 Torr. When the degree of vacuum is low and less than5 Torr, it is difficult to ensure a satisfactory driving force in theproduction of a high-purity steel by the melt process, disadvantageouslyresulting in lowered degassing rate.

In order to more efficiently carry out degassing, preferably, inaddition to the above conditions, when the degree of vacuum in thecourse of evacuation at the time of the degassing reaches the range offrom 5 to 30 Torr, oxygen is reblown (reblowing) in an amount of 0.3 to5 Nm³ per ton of the molten steel preferably for about 2 to 3 min, and,in addition, the flow rate of the gas for agitation during the degassingis regulated to the range of 2.5 to 8.5 Nl/min/t while bringing theamount of the slag 12-1 within the tank during the stop of oxygenblowing to not more than 1.2 tons/m² per unit sectional area of thesteel bath portion in the vacuum tank.

Reblowing of oxygen is carried out from the viewpoint of increasing theconcentration of oxygen in the steel in order to further accelerate theinternal decarburization. At that time, the degree of vacuum is mostpreferably in the range of from 5 to 30 Torr. In this case, when thedegree of vacuum is excessively high and exceeds 5 Torr, the dissolutionof oxygen in the molten steel based on the equilibrium condition becomesdifficult. On the other hand, when oxygen is reblown under a low degreeof vacuum of less than 30 Torr, the blown oxygen is consumed by theoxidation of chromium rather than enrichment of oxygen in the moltensteel.

Further, the amount of oxygen blown at that time is preferably in therange of from 0.3 to 5 Nm³ per ton of the molten steel. When the amountof oxygen reblown is less than 0.3 Nm³/t, oxygen to be consumed in thedegassing is not satisfactorily enriched, even though the degree ofvacuum within the tank at the time of reblowing is in the proper range.On the other hand, when oxygen is reblown in an amount exceeding 5Nm³/t, the oxygen enrichment effect is saturated. In this case, on thecontrary, there is a fear of oxygen being consumed by the oxidation ofchromium.

The reason why the flow rate of the gas for agitation is regulated inthe range of 2.5 to 8.5 Nl/min/t is as follows. In the case of a gasflow rate of less than 2.5 Nl/min/t, the amount of circulated moltensteel is unsatisfactory due to a shortage of agitating force, inhibitingthe promotion of the internal decarburization, which disadvantageouslylowers the degassing rate per se. On the other hand, when the gas flowrate exceeds 8.5 Nl/min/t, the circulation acceleration effect issaturated. On the contrary, an attack on the refractory by the gas isintensified, unfavorably resulting in damage to the refractory.

In addition, preferably, the amount of the slag within the tank duringthe stop of oxygen blowing is brought to not more than 1.2 tons/m² perunit sectional area of the steel bath portion in the vacuum tank. Whenthe amount of the residual slag within the tank exceeds 1.2 tons/m² perunit sectional area of the steel bath portion in the vacuum tank, thecontact between the molten steel surface to be a reaction site in thedecarburization reaction and the high vacuum atmosphere is blocked,resulting in a remarkably lowered area of effective reaction interface.This makes it difficult to maintain the degassing rate on a high level.

In the production of a high-purity stainless steel having a carboncontent of not more than 20 ppm by the melt process, decarburization onthe molten steel surface as a major reaction site in the last stage ofthe degassing should be accelerated. To this end, it is important toensure the activated surface (free surface area of the molten steelsurface which is vigorously agitated by blown gas bubbles) and, at thesame time, to maintain the renewal of the interface in the activatedsurface.

What is particularly important in ensuring the activated surface is tocompletely discharge chromium oxide and slag into the outside of thesnorkel at the time of surface decarburization, because when chromiumoxide or slag produced during the oxygen blowing decarburization is lefteven in a small amount on the activated surface, the surfacedecarburization is inhibited, leading to a lowering in decarburizationrate.

For this reason, during the degassing period, an inert gas should beinjected from the low portion of the ladle which is distant by Hv fromthe molten steel surface within the snorkel (molten still steelsurface), imparting a predetermined agitation intensity K to theactivated surface.

Accordingly, regarding conditions for maintaining the renewal of theinterface in the activated surface and completely discharging chromiumoxide into the outside of the snorkel, as shown in FIG. 18, regulationof the K value defined by the following equation in a range of from 0.5to 3.5 is important:

K=log {S·Hv·Q/P}  (5)

wherein P represents the degree of vacuum, Torr; S represents the gasbubble activated area, m²; Q represents the flow rate of an inert gasblown, Nl/min/t; and Hv represents the distance from the molten steelsurface within the snorkel to the position where the inert gas is blown,m.

In this case, when the K value is smaller than 0.5, the renewal of thegas bubble activated surface and the discharge of chromium oxide areunsatisfactory, resulting in a deteriorated decarburization rate. On theother hand, when the K value exceeds 3.5, the effect of renewal of thegas bubble activated surface is substantially saturated, posing problemssuch as loss of the refractory due to excessively high flow rate of theblown gas.

After the completion of the degassing, if necessary, aluminum forreduction is further introduced to reduce a metal oxide (for example,Cr₂O₃) produced during the oxygen blowing, followed by recovery of themetal.

For example, in the oxygen blowing decarburization refining of astainless steel having a chromium content of not less than 5%,independently of whether the decarburization refining is carried outunder the atmospheric pressure or in vacuo, the oxidation of chromiumcontained in the molten steel, that is, the production of Cr₂O₃, isunavoidable. In this case, after the end of oxygen blowing, a reducingagent should be added to recover the chromium component.

In general, silicon (a ferrosilicon alloy), which exhibits a low heatingvalue in the reduction reaction, is in many cases used as a reducingagent after the oxygen blowing decarburization under the atmosphericpressure. After the oxygen decarburization in vacuo as finish refining,however, when the silicon content of the product is limited, aluminumshould be used as the reducing agent.

When aluminum is used as the reducing agent, however, a thermit reactionrepresented by the following equation (6) occurs. This reaction involvesthe generation of a large amount of heat and necessarily results in atemperature rise in the molten steel.

Cr₂O₃+2 Al→2 Cr+Al₂O₃  (6)

When the molten steel temperature is raised, the equilibrium carbonconcentration in a reduction reaction with carbon contained in themolten steel represented by the following equation (7) is lowered,causing the reaction involving the evolution of a CO gas tosimultaneously proceed:

Cr₂O₃+3 C→2 Cr+3CO↑  (7)

In addition, the equilibrium carbon concentration in the equation (7) isgreatly influenced by the equilibrium partial pressure of CO, that is,the degree of vacuum in operation. The reaction represented by theequation (7) proceeds more significantly with an increase in the degreeof vacuum.

When the violent reaction represented by the equation (7) takes place ina short time, a bumping reaction occurs wherein, with the ascent of theCO gas, the molten steel and the slag are scattered.

Therefore, in order to prevent the reaction involving rapid evolution ofthe CO gas, that is, bumping, it is important to inhibit the progress ofthe reaction represented by the equation (7), that is, to conduct theoperation under a low degree of vacuum of a certain value or less.

When the operation of the reduction is carried out under a low degree ofvacuum, however, the absorption of nitrogen in the molten steel(saturated solubility) is enhanced with an increase in the partialpressure of nitrogen (P_(N2)) within the tank, leading to an increase inthe concentration of nitrogen in the molten steel. Therefore, this isunfavorable in the case of steel species wherein there is a limitationon the nitrogen content.

Thus, in the reduction under a low degree of vacuum, it is veryimportant to simultaneously attain the prevention of bumping and theinhibition of pick-up of nitrogen.

In order to solve this problem, the present invention provides atechnique that solid aluminum, immediately after the introduction ofaluminum, is brought into contact with solid slag to allow the thermitreaction to proceed moderately to form molten slag which covers themolten steel to inhibit the pick-up of nitrogen.

Specifically, the flow rate of the argon gas for agitation during theintroduction of aluminum for reduction is brought to the range of from0.1 to 3 Nl/min/t, and the degree of vacuum is brought to a low value ofnot more than 400 Torr. Thereafter, the pressure is returned to theatmospheric pressure, and the tank is lifted. At the same time, the flowrate of the argon gas for agitation is brought to the range of from 5 to10 Nl/min/t.

Maintaining the flow rate of the argon gas for agitation in the properrange during the introduction of aluminum for reduction and, at the sametime, bringing the degree of vacuum to a low degree of vacuum of notmore than 400 Torr permits the agitation force within the vacuum tank tobe suitably maintained and can inhibit the suspension of the moltensteel and the slag, inhibiting excessive progress of the thermitreaction represented by the equation (6), which can inhibit an extremeincrease in the temperature of the molten steel. Suppression of theagitation during the introduction of aluminum for reduction can inhibitthe dissolution of aluminum in the molten steel and permits a directreaction of aluminum with the slag to improve the reduction rate ofCr₂O₃.

The reason for this is as follows. Previously forming slag in asemi-molten state by direct reduction with aluminum, rather than thedissolution of aluminum directly in the molten steel followed by areduction reaction of the aluminum-containing molten steel with thesolid slag, markedly improves the entanglement (emulsion) of theCr₂O₃-containing slag in the molten steel, resulting in improvedreduction efficiency. Further, melting of the slag in an early stage canoffer a covering effect which prevents the contact between the moltensteel surface and the air. Therefore, the above method is advantageousalso from the viewpoint of the effect of preventing the pick-up ofnitrogen.

In this connection, preferably, the flow rate of the argon gas foragitation in the aluminum introduction period is brought to the range offrom 0.1 to 3 Nl/min/t. When the argon gas flow rate in this periodexceeds 3 Nl/min/t, the thermit reaction represented by the equation (6)excessively proceeds and, at the same time, the emulsion of the slag andthe metal is also intensified, making it difficult to prevent bumping.On the other hand, when the argon gas flow rate is less than 0.1Nl/min/t, the introduced aluminum is deposited within the vacuum tank,often making it impossible to properly introduce aluminum, or otherwisecreating the penetration of the molten steel into a porous plug providedat the bottom of the ladle. This raises an operation problem that, whenthe flow rate is increased in the subsequent stage, a desired flow ratecannot be ensured.

Further, when the degree of vacuum in the aluminum introduction periodis high and exceeds 400 Torr, the agitating force becomes excessive.Specifically, the effective contact area between the slag and the metalis increased, and, in addition, the equilibrium partial pressure of CO,having a close relationship with the degree of vacuum at that time, islowered. This shifts the reaction equilibrium in the equation (7)towards the right side, instantaneously causing significant accelerationof the reaction involving the evolution of the CO gas. This makes itdifficult to prevent bumping.

After the completion of the introduction of aluminum, returning of thepressure to the atmospheric pressure followed by lifting of the vacuumtank and, at the same time, bringing the flow rate of the argon gas foragitation to the range of from 5 to 10 Nl/min/t can suppress theincrease in the molten steel temperature and, in addition, can preventthe progress of the reduction in an early stage and the pick-up ofnitrogen.

Lifting of the vacuum tank permits the reaction zone confined within thesnorkel in the vacuum tank up to that point to be released into thewhole interior of the ladle. Therefore, even though the thermit reactiontakes place, an increase in the temperature of the molten steel is sosmall that the reaction represented by the equation (7) is less likelyto take place. Consequently, the bumping can be avoided. Further,bringing the flow rate of the argon gas for agitation to 5 to 10Nl/min/t after the lifting of the tank can allow the reduction reactionto proceed in an early stage and reduces the concentration of Cr₂O₃ inthe slag to further accelerate the melting of the slag, enhancing thecovering effect exerted by the slag. As a result, the pick-up ofnitrogen can be prevented. When aluminum has been introduced underatmospheric pressure, the tank may be lifted in this state.

In this case, when the flow rate of the argon gas for agitation is lessthan 5 Nl/min/t, the reduction rate of Cr₂O₃ is lowered due to anunsatisfactory agitating force, leading to lowered productivity. On theother hand, when the argon gas flow rate exceeds 10 Nl/min/t, the effectof improving the reduction rate is substantially saturated. Further, inthis case, the covering effect by the slag is reduced because thefluctuation of the molten steel surface is intensified due to theincreased flow rate. This induces the pick-up of nitrogen, abnormaldamage to refractories constituting the ladle, and other unfavorablephenomena.

Further, when a large amount of Cr₂O₃ is produced during the oxygenblowing due to any operation problem during the oxygen blowingdecarburization and, in addition, Cr₂O₃ flows into the outside of thevacuum tank and is deposited and solidified on the upper part of thewall of the ladle, introduction of aluminum into the molten steel isquite unsatisfactory for completely reducing and recovering in a shorttime Cr₂O₃ that has been deposited and solidified on the upper part ofthe wall of the ladle. This is because, in the gas bubbling from the lowportion of the ladle, although the rising of the molten steel around thecenter of the ladle is satisfactory, the rising of the molten steelaround the wall of the ladle is unsatisfactory, resulting in reducedopportunity for the contact of the molten steel with theCr₂O₃-containing slag.

A preferred method for solving this problem is that, immediately afterdegassing, the pressure is returned to the atmospheric pressure, thevacuum tank is lifted, and aluminum is then introduced. Direct contactof aluminum, for reduction, with the slag deposited onto the upper partof the wall of the ladle improves the reduction efficiency of Cr₂O₃.Further, as described above, when a large amount of Cr₂O₃ is producedduring the oxygen blowing, the amount of slag within the vacuum tankinevitably becomes large. In this case, the slag on the upper part ofthe ladle after the lifting of the vacuum tank heaps into a mound.Therefore, when aluminum is added from the top of the ladle, the addedaluminum inevitably advances toward the foot of the mound, permittingaluminum to come into contact with the Cr₂O₃-containing slag around thewall in the upper part of the ladle. As a result, the reduction of Cr₂O₃proceeds, although the reduction reaction takes place between solidphases. Fluctuation of the molten steel by gas additing from the lowportion of the ladle permits the contact of the slag with thehigh-temperature molten steel to be added, accelerating the melting ofthe slag. This further enhances the reduction efficiency of Cr₂O₃.

The present invention will be described in more detail with reference tothe accompanying drawings.

As shown in FIG. 19(A), a snorkel 14 of a straight-barrel type vacuumtank is submerged in a molten steel 11 having a chromium concentrationof not less than 5% contained in a ladle 13. The interior of the snorkel14 is evacuated. In addition, an argon gas as an inert gas for agitationis fed through a porous plug 19 provided at the bottom of the ladle 13into the molten steel while blowing an oxygen gas onto the molten steelfrom above the molten steel within the vacuum tank, thereby carrying outoxygen blowing decarburization refining in vacuo. After the oxygenblowing is stopped, degassing is carried out under a high degree ofvacuum. Thereafter, aluminum 26 for reduction is introduced from abovesolid slag 12-2 to cause the reaction represented by the equation (6),thereby reducing and recovering chromium oxide (Cr₂O₃) produced duringthe oxygen blowing. In this case, the flow rate of the argon gas foragitation during the introduction of aluminum for reduction is regulatedin the range of 0.1 to 3 Nl/min/t, and, in addition, the degree ofvacuum is brought to a low degree of vacuum of not more than 400 Torr.As shown in FIG. 21, this improves the recovery of chromium oxide(Cr₂O₃).

Thereafter, as shown in FIG. 19(B), the pressure of the interior of thesnorkel 14 is returned to the atmospheric pressure, and the snorkel 14is pulled up. At the same time, the flow rate of the argon gas foragitation is increased to the range of from 5 to 10 Nl/min/t. In FIG.19(A), numeral 12-1 designates melted slag, and numeral 12-3 solid slagpresent outside the vacuum tank.

Next, another embodiment of the present invention will be described withreference to FIGS. 20(A) to (C).

Immediately after the oxygen blowing decarburization refining and thedegassing in the same manner as described above, the pressure within thesnorkel 14 is returned to the atmospheric pressure (FIG. 20(A)), and, inaddition, as shown in FIG. 20(B), the snorkel 14 is pulled up. At thesame time, aluminum 26 for reduction is simultaneously introduced. Theflow rate of the argon gas for agitation is regulated in the range offrom 0.1 to 3 Nl/min/t during the introduction of aluminum forreduction.

Slag 12-4 deposited on the upper part of the ladle comes into contactwith the aluminum 26 for reduction, permitting the reduction to proceed.

Subsequently, the flow rate of the argon gas for agitation is increasedto the range of from 5 to 10 Nl/min/t to fluctuate the molten steel asshown in FIG. 20(C), thereby promoting the contact of the solid ordeposited slag with the high-temperature molten steel. This melts theslag and allows the reduction of the slag with aluminum to proceed. Therelationship between the recovery of Cr₂O₃ and the flow rate of theargon gas for agitation in this embodiment is shown in FIG. 22. As canbe seen from this drawing, when the flow rate of the argon gas foragitation is 5 to 10 Nl/min/t, the recovery of Cr₂O₃ can be improvedand, in addition, an increase in pick-up of nitrogen can be prevented.

As described above, in vacuum decarburization refining using a vacuumtank provided with a one-legged, straight-barrel type snorkel, a snorkelin a lower tank of the vacuum tank is submerged in the molten steelwithin the ladle. In this case, for example, the fluidity of the moltensteel, such as molten stainless steel, is large, and high-temperaturerefining, such as oxygen blowing decarburization, is carried out. Thiscauses refractories constituting the snorkel to undergo melt loss due tothe flow of the molten stainless steel created by oxygen blowing oragitation, or otherwise causes the refractories to be worn by spallingor the like due to a rapid temperature change involved in the transferfrom the refining period to the standing period.

The wear of the refractories constituting the snorkel leads to alowering in rate of operation of the vacuum refining apparatus, and thelowered throughput capacity in the vacuum refining makes it impossibleto treat the object steel species. As a result, the production of highgrade steels per se becomes difficult.

On the other hand, the wear of the snorkel used in the vacuum refiningin an early stage leads to increased cost of refractories constitutingthe snorkel, and a lot of time and labor are required in the replacementof the vacuum tank and the snorkel.

According to the present invention, the above problem has been solved byimmersing the snorkel, after the completion of the refining, in slaghaving a regulated composition to coat the slag onto the surface of thesnorkel.

Specifically, the slag after the completion of the refining underreduced pressure is regulated so as to comprise 55 to 90% by weight intotal of Al₂O₃ and CaO, 1 to 10% by weight of Cr₂O₃, and 7 to 25% byweight of SiO₂ with the balance consisting of 2 to 10% by weight of atleast one member selected from FeO, Fe₂O₃, and MgO.

In the above composition of the slag, when the total amount of Al₂O₃ andCaO is less than 55% by weight, the slag coating on the snorkel has poorcorrosion resistance and, in this case, the effect of protecting thesnorkel cannot be attained by the slag coating. On the other hand, whenthe total amount of Al₂O₃ and CaO exceeds 90% by weight, the meltingpoint of the slag becomes high and slagging is poor. This makes itdifficult to coat the slag onto the snorkel and is an obstacle to thereduction of the chromium oxide in the reduction refining as theprevious step.

When the Cr₂O₃ content is less than 1% by weight, the anticorrosioneffect derived from the formation of a highly viscous material uponreaction with slag or the like is lowered. On the other hand, when theCr₂O₃ content exceeds 10% by weight, slagging is poor, making itdifficult to coat the slag onto the snorkel.

When the content of SiO₂ in the slag composition formed upon completionof the reduction refining is less than 7% by weight, the slag haslowered viscosity and higher melting point. In this case, as with thecase of increased total amount of Al₂O₃ and CaO, the slagging is poor,and coating becomes difficult.

When the SiO₂ content exceeds 25% by weight, the melting point of theslag is significantly lowered, making it impossible to form asatisfactory coating protective layer.

In the slag composition, FeO, Fe₂O₃, and MgO as the balance are producedin the refining under reduced pressure and included in the previousstep, and the slag contains 2 to 10% by weight of at least one memberselected from FeO, Fe₂O₃, and MgO. When the amount of FeO, Fe₂O₃, andMgO is increased, the corrosion resistance of the slag is lowered due toa lowering in melting point. In particular, when the amount of MgO isless than 2% by weight, the melt loss of refractories constituting thesnorkel is significant, while when the amount exceeds 10% by weight, MgOshould be additionally added.

In the composition of slag 12, which has been finally formed through theabove steps, SiO₂ comprises a slag component (the content of SiO₂ in theslag included: 30% by weight) included at the time of tapping of themolten steel 11 from a decarburization refining furnace (not shown),such as a converter, into the ladle 13, and Si (0.03 to 0.20% by weight)contained in the molten steel 11 before the decarburization refiningunder reduced pressure.

The SiO₂ content can be previously determined by analysis. The wholeamount of Si in the molten steel 11 is expressed in terms of SiO₂, andthe total of both the SiO₂ contents is regarded as the SiO₂ content.

The SiO2 content in terms of the total of both the above contents isregulated in the range of from 7 to 25% by weight by regulating any oneof or both the amount of the inflow slag and the amount of silicon addedto the molten steel 11.

The amount of CaO to be added in the degassing refining is determinedfrom the amount of chromium oxide and the like to be reduced in thereduction refining by the following method.

At the outset, the amount of chromium oxide produced is predicted fromthe above-described decarburization refining conditions, that is, theamount of blown oxygen and the attained final carbon concentration.Alternatively, a method may be used wherein the molten steel or slag isanalyzed, and the amount of metallic aluminum to be added for reducingthe amount of the produced chromium oxide and, in addition, the amountof Al₂O₃ produced are determined according to the equation (8):

Cr₂O₃+2 Al→Al₂O₃+2 Cr  (8)

The amount of CaO is determined from the amount of Al₂O₃, and regulationis carried out so that the total amount of CaO and Al₂O₃ is 55 to 90% byweight. The regulation of CaO and Al₂O₃ may be made by varying theamount of both or any one of CaO and Al₂O₃ added.

The amount of Cr₂O₃ is determined by the amount of metallic aluminumadded in the reduction refining, and decreases with increasing theamount of the metallic aluminum added. Therefore, the amount of Cr₂O₃ isregulated in the range of from 1 to 10% by weight.

In the composition constituting the slag 12, FeO, Fe₂O₃, and MgO as thebalance are produced in the refining under reduced pressure and includedin the previous step. The amount of slag included, the amount ofmetallic aluminum added in the reduction refining and the like areregulated so that the slag contains 2 to 10% by weight of at least onemember selected from FeO, Fe₂O₃, and MgO.

The Al₂O₃/CaO ratio in the slag is brought to the range of from 0.25 to3.0.

In the slag, after the refining under reduced pressure, wherein thetotal content of Al₂O₃ and CaO is in the range of from 55 to 90% byweight, when the Al₂O₃/CaO ratio is less than 0.25, phase transformationoccurs upon cooling of the slag, causing the slag to crumble anddisintegrate, which results in separation of the slag coating.

On the other hand, when the Al₂O₃/CaO ratio exceeds 3.0, slagging ispoor, rendering coating of the snorkel with the slag difficult.

Coating of the slag 12 regulated in each refining onto the snorkel 14will be described with reference to FIG. 23 showing the structure of thesnorkel 14.

Regulated slag 12 after each refining and refining under reducedpressure is melted at a temperature of 1650 to 1750° C.

Regarding the snorkel 14 which is submerged in the slag 12 and themolten steel 11, upon the completion of the refining under reducedpressure, the pressure of the interior of the vacuum tank 15 and thesnorkel 14 are returned to the atmospheric pressure. The snorkel 14, thepressure of which has been returned to the atmospheric pressure, islifted above the slag 12 and then stands by. At the point of timeimmediately after lifting of the snorkel, both the temperature ofchromia-magnesia bricks 28 constituting the inside of the snorkel 14 andthe temperature of high alumina, prepared unshaped refractories 29constituting the outside of the snorkel 14 are substantially the same asthe temperature of the slag 12, that is, 1650 to 1750° C. Thetemperature is lowered to 1200 to 1300° C. by the standing-by of thesnorkel 14 in the lifted state for about 0.5 to 1 min. Next, the snorkelis submerged in the slag 12 layer by 270 to 530 mm from the front end ofthe snorkel 14, and, immediately after that, the snorkel 14 is slowlylifted to form a 30 mm-thick coating 32.

After the formation of the coating 32, the snorkel 14 is further allowedto stand by for additional 5 min. When the temperature of the surface ofthe coating 32 has reached about 800° C., the snorkel 14 is submerged inthe molten steel 11 within the next ladle 13, followed by the nextrefining under reduced pressure. Thereafter, the formation of thecoating 32 on the snorkel 14 and the refining under reduced pressure arerepeatedly carried out.

After the formation of a 30 mm-thick coating, the snorkel may be againsubmerged in the slag 12 and allowed to stand by, thereby forming a 60mm-thick coating.

The coating 32 formed by double coating procedure has the effect ofpreventing both breaking and melt loss of refractories derived fromspalling created by a rapid temperature change from 1750° C. to theatmospheric temperature, or from 800° C. to the temperature of themolten steel 11 around 1750° C. at the time of immersion of the snorkelin the molten steel.

The bricks 28, 29 constituting the snorkel 14 are held by a core metal27 provided with a flange 31, and the prepared unshaped refractory brick29 is held by a stud 30.

Next, an apparatus which is most preferred in practicing theabove-described vacuum degassing refining method will be described.

While the method according to the present invention can preventsplashing per se created during decarburization refining, the apparatusof the present invention is characterized by means that, when dust andthe like are created, can trap and melt the dust in the vacuum tank and,also when a dust-containing gas is introduced into an evacuation duct,can inhibit the deposition and accumulation of the dust, and, inaddition, can prevent damage to refractories constituting the lower tankin the vacuum tank caused by heat of radiation from the molten steel(mainly from a hot spot) during the vacuum refining.

A vacuum decarburization refining apparatus according to one embodimentof the present invention will be described.

As shown in FIGS. 24 to 26, a vacuum decarburization refining apparatus10 comprises: a ladle 13 that is provided, at the bottom thereof, withan inert gas blowing nozzle 19 and contains a molten steel 11; a vacuumtank 15 provided with a snorkel 14, submerged in the molten steel 11within the ladle 13, and an evacuation hole 16-1 connected to anevacuation apparatus (not shown); and an oxygen lance 18 that isliftably provided in a canopy 35 of the vacuum tank 15.

The above elements constituting the vacuum decarburization refiningapparatus will be described in more detail.

The ladle 13 is a substantially cylindrical iron container, and theinner wall in contact with the molten steel 11 is lined with arefractory, for example, an alumina-silica or alumina-zircon refractory.

The molten steel 11 within the ladle 13 is agitated by an ascending, andthe kinetic energy of, an inert gas blowing into the molten steel 11through a gas blown nozzle 19 provided in the ladle 13, therebyenhancing the vacuum refining reaction in the molten steel 11.

The vacuum tank 15 is a container for vacuum refining that is mainlylined with a refractory brick such as a magnesia-chromia brick (a partof the container may be constituted by a prepared unshaped refractory).The vacuum tank 15 comprises an upper tank 33 and a lower tank 34, thelower end of the lower tank serves as a snorkel 14 and is submerged inthe molten steel.

When the vacuum tank is evacuated, the molten steel ascends through thesnorkel, permitting a molten steel surface 11-1 different from themolten steel surface within the ladle 13 to be formed within thesnorkel. An oxygen gas is blown against the surface through the lance.

In the present invention, the snorkel refers to a lower end portion ofthe vacuum tank which is located below the position, of the vacuum tank,where the uppermost surface of the sucked molten steel is in contactwith the vacuum tank.

The snorkel 14 is in a substantially cylindrical form having an innerdiameter D_(F), and the snorkel 14, particularly in its portion which issubmerged in the molten steel 11 and through which the molten steelascends, is coated with a prepared unshaped refractory, for example, analumina-silica, by casting. When a splash is scattered from the surfaceof the molten steel within the snorkel 14 in the same density, theamount of the splash decreases with reducing the sectional area of thesnorkel. Therefore, the inner diameter of the snorkel is minimized whiletaking into consideration the decarburization efficiency.

The present invention is characterized by providing a larger-diametersection 36, having an inner diameter D_(L) larger than the innerdiameter D_(F) of the snorkel and having a length A in the verticaldirection, in the lower tank 34 continued to the snorkel 14. Thelarger-diameter section serves to disperse a splash created by an oxygenjet gas blown through the oxygen lance 18 against the molten steelsurface 11-1 and, at the same time, to reduce the thermal influence of ahot spot created by the oxygen jet gas or heat of radiation from themolten steel surface 11-1 on the side wall section of the vacuum tank,and is a constituent element important to the vacuum tank of the presentinvention.

The inner diameter D_(L) of the larger-diameter section is specified, inrelation with the position of a gas blown hole of the oxygen lance 18,so that the ratio of the inner diameter D_(L) to the oxygen gas blowingdistance L (distance between the lower end of the oxygen lance and themolten steel surface 11-1), D_(L)/L, is in the range of from 0.5 to 1.2.This offers the above effect.

Further, a smaller-diameter section (a diameter-reduced section) 37having an inner diameter Ds is provided, at a position a vertical lengthA from the lower end of the larger-diameter section 36, connected to thelarger-diameter section 36. The smaller-diameter section 37 functions toinhibit the introduction of splash or dust into the upper tank in thevacuum tank, and melts dust and the like, deposited on the bottom facethereof, by heat of radiation from the molten steel surface to removethe dust and the like from the smaller-diameter section. For thisreason, in order that the smaller-diameter section 37 attains the aboveeffect, the relationship between the inner diameter Ds of thesmaller-diameter section and the inner diameter D_(L) of thelarger-diameter section, that is, the relationship between the sectionalarea Ss of the space As of the smaller-diameter section and thesectional area S_(L) of the space A_(L) of the larger-diameter section,is important. According to the present invention, the ratio S_(S)/S_(L)is specified to the range of from 0.5 to 0.9. Further, thesmaller-diameter section is provided at a position against which astream of the oxygen gas blown through the lance does not directlyimpact and where melt loss of the refractory derived from the heat ofradiation from the hot spot and the molten steel surface does not occurand only the dust deposited onto the refractory can be remelted (forexample, at a position where the surface temperature of the refractoryconstituting the smaller-diameter section is 1200 to 1700° C.). In thiscase, the length A is specified to be 1 to 3 m.

The difference between the inner diameter D_(s) of the smaller-diametersection and the outer diameter of the oxygen lance 18 in the radialdirection is preferably small. When the difference is excessively small,the exhaust gas passage becomes so narrow that the decarburizationefficiency lowers. Therefore, the difference d is preferably in therange of from 100 to 300 mm.

Specifically, in the decarburization refining in vacuo, like the presentinvention, the melt loss of the refractory in the side wall section ofthe vacuum tank (freeboard section) not directly submerged in the moltensteel 11 is governed by the surface temperature of the refractory, thetemperature of the atmosphere gas, and the flow rate of a gas thatcollides with the working face of the refractory.

Therefore, in order to prolong the service life of the refractory in thefreeboard section, it is important to maximize the distance of therefractory from a high-temperature hot spot created by oxygen blowingand a decarburization reaction and to reduce the flow rate of the gaswhich collides with the working face of the refractory.

In the impinging region (the hot spot) in which a jet stream of theoxygen gas blown from the oxygen lance 18 impinges with the molten steel11, carbon contained in the molten steel is oxidized with the oxygen gasto evolve a CO gas and the temperature in the vicinity of the hot spotis as high as about 2400° C. due to the calorific value involved in thedecarburization reaction.

Further, a secondary combustion reaction occurs wherein the evolved COgas is burned in the atmosphere (CO+(½)O₂→CO₂). Therefore, the gastemperature (atmosphere temperature) at a portion just above the hotspot becomes very high.

The CO gas flow rate also becomes maximum at the portion just above thehot spot immediately after the evolution of the CO gas.

Thus, the freeboard section in the vacuum decarburization refiningundergoes wearing action due to heat of radiation, a gas stream or thelike which occurs by the hot spot having a high-temperature and theportion just above the hot spot. Therefore, it is important to properlymaintain geometrical arrangement between the hot spot and the freeboardsection.

According to this embodiment of the present invention, setting of thegeometrical arrangement between the hot spot and the refractory of thevacuum tank in the above manner can minimize the melt loss of therefractory in the freeboard section, the oxygen lance and the like and,at the same time, can prevent the introduction of dust created bysplashing of the molten steel 11 into the evacuation system, realizingthe operation of vacuum decarburization refining with high productivity.

Next, a vacuum decarburization refining apparatus according to anotherpreferred embodiment of the present invention will be described.

As shown in FIGS. 27 to 29, the construction of a vacuum decarburizationrefining furnace 10 according to the second preferred embodiment issubstantially the same as that according to the first preferredembodiment, except that the structure of the smaller-diameter section 37of the vacuum tank 15 in the vacuum decarburization refining apparatus10 described in the first preferred embodiment has been changed to thestructure of fan-shaped shields 38, 39, 40. Therefore, like parts havethe same index numerals, and detailed description thereof will beomitted.

As shown in FIG. 27, the fan-shaped shields 38-40 are provided so as tobe different from one another in position as well as in level in thevertical direction. Further, as shown in FIG. 29, the shields areprovided at a fan angle θ for covering the whole molten steel surfacewithin the vacuum tank except for the sectional area Ss in the space Asdefined by the shields.

As shown in FIG. 28, regarding the fan-shaped shields 38 to 40, forexample, the fan-shaped shield 38 is provided by fixing a core metal 41,with a cooling air passage 43 provided therein, onto the inner side ofan iron skin 15-1 in the vacuum tank and fixing a prepared unshapedrefractory, such as alumina castable refractory, onto the core metal 37through a Y-shaped stud 42 mounted on the core metal 41.

Thus, provision, as the smaller-diameter section, of a plurality offan-shaped shields so as to be different from one another in level caneffectively shield the heat of radiation from the hot spot on the moltensteel surface 11-1, and splash and, in addition, enables vacuumdecarburization refining while maintaining the evacuation passage in thevacuum tank 15 so as not to avoid an increase in evacuation resistance.

In this preferred embodiment, the formation of the fan-shaped shieldusing a prepared unshaped refractory has been described. It is alsopossible to form the fan-shaped shield using a shaped refractory, forexample, a magnesia-chromia refractory brick.

The fan angle θ in each fan-shaped shield may not be necessarilyidentical so far as the whole molten steel surface except for the spacearound the oxygen lance is covered with the surface of the fan-shapedshields. Further, the number of fan-shaped shields is not limited tothree.

Furthermore, no operation problem occurs when the fan-shaped shieldsrespectively in their surfaces facing the molten steel surface partiallyoverlap with each other or one another. This also falls within the scopeof the present invention.

FIGS. 27 and 28 shows such a state that blowing is carried out under alow degree of vacuum within the vacuum tank. Therefore, in this state,the height of the surface of the molten steel within the snorkel is low.

In the vacuum tank having the above structure according to the presentinvention, a space is provided in the smaller-diameter section so thatthe oxygen nozzle 18 is passed through the space. Therefore, there is apossibility that an exhaust gas containing dust ascends through thespace, reaches the side wall of the upper tank in the vacuum tank,particularly the canopy and the side wall near the canopy, causing thedust to deposit and accumulate.

The present invention further provides means for preventing thedeposition of the dust.

Specifically, as shown in FIGS. 24 and 30, burners 44-1, 44-2 areprovided so that the front end thereof is located below the canopy 35 bya distance F (burner front end distance F). In this case, these burnersare inserted and provided in the upper tank 33 so as to face each otherso that the gas ejection direction has a predetermined burner ejectionangle θh to the vertical direction and a burner whirling angle θr.

The burner front end distance F is preferably in a range of from 0.3 to3 m, the burner ejection angle θh is preferably in a range of from 20°to 90°, and the whirling angle θr is preferably in a range of from 15°to 30°.

By virtue of the construction of the burners, an oxygen gas, a fuel gas,or a mixed gas composed of the oxygen gas and the fuel gas blown throughthe burners 44-1, 44-2 into the upper tank 33 forms a whirling streamwithin the upper tank 33, permitting a refining gas evolved in thecourse of the oxygen blowing refining to be efficiently mixed with theoxygen gas, fuel gas and the like and, at the same time, permitting thetemperature of the canopy 35 to be properly held.

Specifically, the above burners are applied during the oxygen blowingdecarburization refining, the surface temperature of the canopy isdetected with a plurality of thermocouples buried in the canopy 35, andthe surface temperature of the canopy is kept in a range of 1200 to1700° C. as shown in FIG. 31. In this case, an inspection hole formeasurement of the temperature may be provided in the side wall of theupper tank so that the surface temperature of the canopy is directlymeasured with an optical pyrometer. The dust, which has reached aroundthe canopy, is melted and removed, preventing a lowering in yield ofchromium or iron derived from the deposition of the dust.

In the subsequent non-oxygen blowing refining period, the blowing of theoxygen gas through the oxygen lance 18 is ended, and an argon gas isinjected from the low portion of the ladle 13 into the molten steel 11to agitate the molten steel 11 in the snorkel 14.

This can homogenize the remaining refining reaction, the molten steeltemperature, and the constituents of the molten steel.

Therefore, also in the non-oxygen blowing refining period, theaccumulation of dust, onto the canopy 35, produced by the agitation ofthe molten steel and the evacuation of the interior of the snorkel 14 byan evacuating apparatus can be prevented.

In the standing-by period, the evacuating apparatus is stopped, thepressure within the snorkel 14 is returned to the atmospheric pressure,and the lower end of the snorkel 14 is pulled up from the molten steel11 in the ladle 13 and is held in a standing-by state. During thisperiod, the surface temperature of the canopy is regulated in apredetermined temperature range (1200 to 1700° C.) using the burners44-1, 44-2.

In the standing-by period, use of air instead of the oxygen gas forburning the fuel gas is preferred from the viewpoint of cost and, inaddition, avoiding damage to the refractory by oxidation.

Thus, even though dust is accumulated on the canopy 35 or a portionaround the canopy 35, it can be melted and allowed to flow down andremoved. In addition, it is possible to effectively prevent the damageto the refractory of the snorkel 14 due to thermal stress, created byexcessive thermal shock in the initiation of the subsequent oxygenblowing refining period.

In the present invention, when the vacuum decarburization refining iscarried out, the degree of vacuum within the vacuum tank is maintainedat a predetermined value while sucking an exhaust gas evolved during therefining through a steam ejector. In this case, the sucked exhaust gasis cooled by means of a gas cooler and fed into an exhaust gas treatmentsystem.

Therefore, there is a possibility that the dust contained in the exhaustgas is sucked, together with the exhaust gas through a duct, and, asshown in FIG. 35, the dust is deposited and accumulated within the ductto inhibit the flow of the exhaust gas.

Accordingly, the present invention further provides a vacuum refiningapparatus that can prevent clogging of an evacuation duct with dustintroduced into the evacuation duct, permitting the attained degree ofvacuum within the vacuum tank to be maintained on a predetermined leveland, in addition, can facilitate the removal of dust.

The present invention will be described with reference to FIGS. 32 to34. As shown in the drawing, in an exhaust gas treating apparatus usedin the vacuum refining apparatus 10, an evacuation duct 16-1 is providedin the upper tank of the vacuum tank 15, and an duct inlet 45 isconnected to an inlet of a gas cooler 55 for cooling the exhaust gasthrough the duct.

A dust pot 53 for collecting the dust contained in the exhaust gas isprovided in the course of the passage of the evacuation duct 16-1 havingan actual length L₀ of about 15 to 50 m, and the evacuation ductextending from the upper tank to the dust pot is constructed so that thedust is not accumulated within the evacuation duct.

Specifically, as shown in FIG. 32, the evacuation duct 16-1 leading tothe dust pot 53 comprises an ascendingly inclined section 46, having atotal length of about 1.5 m, inclined upward from the duct inlet 45 atan inclination angle (θ₀) of 30° to 60°, and a descendingly inclinedsection 48, having a total length of about 1.5 m, inclined downward fromthe top 47 of the ascendingly inclined section 46 at an inclinationangle of about 45°.

When the upward inclination angle is less than 30°, this angle issmaller than the angle of repose of a powder constituted by dustcontained in the exhaust gas. This causes the dust, which has reachedthe ascendingly inclined section, to be gradually accumulated withoutslipping down into the vacuum tank.

On the other hand, the adoption of an inclination angle exceeding 60° isdifficult from the viewpoint of a design due to the restriction of thesystem. Further, when the inclination angle exceeds 60°, the effect ofdropping the dust on the ascendingly inclined section into the vacuumtank is substantially saturated. For this reason, the upper limit of theinclination angle is 60°.

The actual length L₀ of the evacuation duct refers to the length of theevacuation duct along the evacuation direction, that is, the totallength from the duct inlet to the gas cooler.

When the actual length is less than 15 m, the amount of the dust in theexhaust gas introduced from the vacuum tank into the gas cooler isremarkably increased and, at the same time, the exhaust gas temperaturebecomes so high that the load of the gas cooler is unfavorablyincreased.

On the other hand, when the actual length exceeds 50 m, the load imposedon the evacuating apparatus is beyond a limit, making it difficult toattain the necessary degree of vacuum.

A heating device 49 is provided aslant toward the ascendingly inclinedsection 46 around the top 47 of the ascendingly inclined section 46 sothat dust and the like accumulated on the top 47, the ascendinglyinclined section 46, or the descendingly inclined section 48 areheat-melted and flow down into the vacuum tank 11 or the dust pot 53.

A branched section 50 is provided below the descendingly inclinedsection 48, and the dust pot 53 is detachably disposed at the lower partof the branched section 50 so that the dust and the like dropped alongthe inside of the inclined duct in the descendingly inclined section 48are collected in the dust pot 53.

As shown in FIG. 33 (plan view), the evacuation duct 16-1 is constructedso that the flow direction of the exhaust gas is changed by about 90° inthe branched section 50. Changing the direction and speed of the exhaustgas in this way can accelerate the dropping of the dust contained in theexhaust gas into the dust pot 53.

The body of the evacuation duct 16-1 further extends, from the endportion of the descendingly inclined section 48 as the branched section50 located just above the dust pot 53, through a curved portion and alinear portion to an inlet of the gas cooler 55.

The system is constructed so that the actual length (L₀) of theevacuation duct 16-1 extending from the duct inlet 45 to the inlet ofthe gas cooler 55 and the inclination angle (θ₀) are if necessary set asdesired.

The gas cooler 55 is a cooling device, for an exhaust gas, with acooling plate or the like provided therein, and is constructed so thatthe gas within the cooler is discharged by means of an evacuationapparatus (not shown). Solid particles (dust) in the exhaust gas, whichhave collided against the cooling plate or the inner wall of the coolerand consequently lost speed, are collected in a inverted conical lowerpart of the gas cooler 55 and hence may be recovered according to need.

As shown in FIG. 34, a pot detaching device 52 comprises: a guide rod 58having in its front end a cotter hole 57; a hydraulic cylinder 60 forvertically moving the guide rod 58 through a disc spring 59; an upperflange 63 for fixing the hydraulic cylinder 60; and a fixed flange 61for movably holding the guide rod 58 through a guide hole (not shown)for connection to a receiving flange 62 of the dust pot 53.

The dust pot 53 is a substantially cylindrical container, having abottom section, made of steel or a casting and comprises: a receivingflange 62 disposed in the upper end portion; a guide rod insertion holefor inserting therein the guide rod 58 of the pot detaching device 52provided in the receiving flange; and a pair of suspension trunnions 54provided, so as to face each other, in the outer periphery of the dustpot 53.

The dust pot 53 is constructed so that, if necessary, the inner wall maybe covered with a refractory lining material, such as a castablerefractory lining material.

When a large amount of dust has been collected in the dust pot 53, thedust pot 53 may be detached using the pot detaching device 52,permitting the dust collected in the dust pot 53 to be easily removedand, at the same time, enabling maintenance, such as cleaning around thebranched section 50, to be carried out.

The dust pot 53 may be detached from the evacuation duct 16-1 asfollows. At the outset, a chain 65 is mounted on a metal hanger 64mounted on the trunnion 54 of the dust pot 53, and the dust pot 53 issupported by means of a chain block (not shown). In this state, fixingbolt and nut between the receiving flange 62 and the fixing flange 61are removed.

Next, the hydraulic cylinder 60 is operated using a hydraulic unit (notshown) to depress the guide rod 58 while pressing the disc spring 59.

This permits the force of constraint, applied to the cotter 56, to bereleased, and the cotter 56 inserted in the cotter hole 57 of the guiderod 58 can be removed.

The cotter 56 is removed from the cotter hole 57, and, in addition, thedust pot 53 is lowered using the chain block.

In this way, the guide rod 58 may be pulled out from the guide rodinserting hole 62-1 of the receiving flange 62 to completely separatethe dust pot 53 from the evacuation duct 16-1, followed by removal ofthe dust, containing metal and the like, collected in the dust pot 53.

As described above, the evacuation duct of the present invention caneffectively prevent dust from accumulating within the duct. Therefore, apredetermined degree of vacuum can be maintained without increasing thepressure loss involved in evacuation of the evacuation duct.

The apparatus of the present invention has at least one of the abovefeatures, realizing stable operation of the vacuum refining apparatus.

EXAMPLES Example 1

In this example, vacuum oxygen blowing refining of a stainless steelaccording to one embodiment of the present invention was carried outusing a vacuum oxygen blowing refining apparatus on a scale of 150 tons.

In a converter, a molten steel having [% C] 0.6 to 0.7% and [% Cr] 10 to20% was prepared by the melt process, and temperature elevation andoxygen blowing decarburization were carried out using an oxygen blowingrefining apparatus shown in FIG. 1.

In this case, the oxygen blowing rate was regulated in such a mannerthat, for all the cases independently of the temperature elevationperiod and the decarburization refining period, the oxygen blowing ratewas kept at a constant rate of 23.3 Nm³/h/t until [% C] reached 0.3%;when [% C] was in the range of from 0.15% to 0.05%, the oxygen blowingrate was reduced from 23.3 Nm³/h/t to 10.5 Nm³/h/t at a constant rate;and when [% C] reached 0.05%, the blowing of oxygen was stopped. Theflow rate of an argon gas for agitation was evenly 4.0 Nl/min/t for thetemperature elevation period and 2.7 Nl/min/t for the decarburizationrefining period.

Conditions and results for runs according to Example 1 of the presentinvention are given, in comparison with comparative runs, in Table 1 andFIG. 4. Run Nos. 1 to 5 fall within the scope of the present invention,and run Nos. 6 to 11 are comparative runs.

For run Nos. 1 to 5 according to the present invention, as shown in FIG.4, since both the G value for the aluminum temperature elevation periodand the G value for the decarburization refining period satisfy theformula (1), in the temperature elevation period and the decarburizationrefining period, the amount of chromium oxidized and the amount ofsplashing were very small.

On the other hand, in run No. 6 wherein the G value in the aluminumtemperature elevation period was larger than −20 on the average, theoxidation of chromium significantly proceeded in the temperatureelevation period. Run No. 7 is a run where, although the G value in thealuminum temperature elevation period was not more than −20 on theaverage, it exceeded −20 (maximum value −18) during the temperatureelevation period. In this run, the oxidation of chromium proceeded inthe period where the G value exceeded −20.

In run No. 8 where the average G value (−18) in the decarburizationrefining period exceeded −20, the oxidation of chromium excessivelyproceeded. On the other hand, run No. 9 is a run where although theaverage G value (−24) was in the range of from −20 to −35, it exceeded−20 in a part of the decarburization refining period. In this run, theoxidation of chromium proceeded during this period. In run No. 10 wherethe G value (−37) was less than −35 in a part of the decarburizationrefining period, splashing was significantly created in this periodposing a problem of deteriorated operation, although the oxidation ofchromium was prevented. In run No. 11 where aluminum for an increase intemperature was introduced at once during the temperatureelevation/oxygen blowing period, the oxidation of chromium was increasedin the temperature elevation period.

In run No. 4, according to the present invention, the G value in thedecarburization refining period was regulated as specified in Table 1(2). Specifically, decarburization refining was carried out in such amanner that in the course of the decarburization wherein [% C] of themolten steel was decreased from 0.7% to 0.05% (at the time of stoppingof the oxygen blowing), [% Cr] and T were determined, and, based on thedata, P within the vacuum tank was regulated to regulate the G value asshown in Table 1 (2). In the refining, as indicated in Table 1 (2), gooddecarburization results could be obtained when the regulation wascarried out so that, for the G value, the maximum value was −21 with theminimum value being −25 and the average value being −23.

TABLE 1 G value during G value in Amount of Cr oxidized, Al temp.decarburization Introduction kg/t elevation refining period of Al Temp.Decarbu- Run Aver- Aver- for temp. elevation rization Splash- Evalu- No.age Max. Min. age Max. Min. elevation period period Total ing ation Inv.1 −25 −22 −27 −28 −27 −30 Dividedly 0.2 0.7 0.9 Slight ◯ 2 −23 −21 −25−27 −25 −31 Dividedly 0.3 0.8 1.0 Slight ◯ 3 −22 −20 −24 −25 −23 −29Dividedly 0.5 0.9 1.4 Slight ◯ 4 −22 −21 −23 −23 −21 −25 Dividedly 0.41.1 1.5 Slight ◯ 5 −26 −21 −28 −30 −25 −35 Dividedly 0.2 0.4 0.6 Slight◯ Comp. 6 −16 −15 −17 −27 −25 −29 Dividedly 2.4 0.7 3.1 Slight X 7 −21−18 −23 −24 −22 −26 Dividedly 2.1 0.9 3.0 Slight X 8 −22 −20 −24 −18 −15−26 Dividedly 0.5 4.6 5.1 Slight X 9 −24 −23 −25 −24 −18 −29 Dividedly0.3 2.7 3.0 Slight X 10  −22 −21 −25 −29 −26 −37 Dividedly 0.5 0.2 0.7Severe X 11  −23 −21 −26 −27 −25 −29 At one time 2.7 0.4 3.1 Slight XNo. G pTorr T^(k) C, % Cr, % 1 −21 160  1630 0.7 16.3 2 −22 130  16500.5 16.3 3 −24 80 1670 0.3 16.2 4 −25 30 1690 0.1 16.1 5 −25 20 1720 0.05 15.9

Example 2

In order to demonstrate the effect attained by adding CaO, the procedureof Example 1 was repeated, except that CaO together with aluminum wasintroduced during the aluminum temperature elevation period.

Runs according to the present invention, together with comparative runs,are shown in Tables 2 and 3. Run Nos. 1 to 12 are runs according to thepresent invention. On the other hand, for run No. 13, since theW_(cao)/W_(Al) ratio was less than 0.8, the production of calciumaluminate was not accelerated, causing slag to remain solidified, whichmade it difficult to sample the molten steel and at the same timeresulted in deteriorated oxygen efficiency in decarburization. In runNo. 14, due to excessive CaO, the amount of slag was so large that thedecarburization by oxygen jet in the decarburization period wasinhibited. Run Nos. 15 and 16 are runs where the immersion depth of thesnorkel in the temperature elevation period was less than 200 mm andexceeded 400 mm. A immersion depth of less than 200 mm made it difficultto sample the molten steel and at the same time resulted in loweredoxygen efficiency in decarburization. On the other hand, when theimmersion depth exceeded 400 mm, the oxygen efficiency indecarburization was lowered due to unsatisfactory discharge of the slagwithin the tank (that is, due to inhibition of decarburization caused bycovering), although the molten steel could be easily sampled. Run Nos.17 and 18 are runs where the immersion depth of the snorkel in thedecarburization period was less than 500 mm and exceeded 700 mm. Whenthe immersion depth was less than 500 mm, solidification of slag(difficulty of sampling the molten steel) due to outflow of Cr₂O₃-richslag into the outside of the snorkel in an early stage and lowering inoxygen efficiency in decarburization were observed. On the other hand,when the immersion depth exceeded 700 mm, the oxygen efficiency indecarburization was unfavorably lowered due to worsening of circulationof the molten steel. Nos. 19 and 20 are runs where the flow rate of anargon gas for agitation in the temperature elevation period was lessthan 3.3 Nl/min/t and exceeded 4.7 Nl/min/t. When the flow rate of theargon gas was less than 3.3 Nl/min/t, the oxygen efficiency indecarburization was deteriorated and attributable to the occurrence of alarge amount of residual slag within the tank. On the other hand, whenthe flow rate of the argon gas exceeded 4.7 Nl/min/t, it becamedifficult to sample the molten steel due to unsatisfactory production ofcalcium aluminate. Run Nos. 21 and 22 are runs where the flow rate ofthe argon gas for agitation in the decarburization period was less than1.7 Nl/min/t and exceeded 6.0 Nl/min/t. When the flow rate of the argongas was less than 1.7 Nl/min/t, the oxygen efficiency in decarburizationwas deteriorated due to unsatisfactory circulation, while when the flowrate exceeded 6.0 Nl/min/t, the oxygen efficiency in decarburization wasdeteriorated due to the outflow of the produced Cr₂O₃ into the outsideof the snorkel in an early stage.

TABLE 2 Flow rate of Ar gas for Oxygen efficiency Immersion depth, mmagitation, Nl/min/t in decarburization Run W_(CaO)/ Temp. eleva-Decarburiza- Temp. eleva- Decarburiza- in decarburization Sam- Evalu-No. W_(A1) tion period tion period tion period tion period period, %pling ation Inv. 1 1.0 300 600 4.0 2.7 75 ◯ ◯ 2 1.4 350 650 3.7 2.3 73 ◯◯ 3 0.8 300 600 3.9 2.5 71 ◯ ◯ 4 4.0 300 600 3.8 4.3 70 ◯ ◯ 5 1.5 200600 4.2 2.9 74 ◯ ◯ 6 1.1 400 650 3.5 3.2 71 ◯ ◯ 7 1.7 300 500 3.8 5.4 75◯ ◯ 8 2.6 250 700 4.1 3.1 73 ◯ ◯ 9 1.5 350 550 3.3 2.6 70 ◯ ◯ 10 3.4 300600 4.7 3.3 72 ◯ ◯ 11 1.2 300 600 3.9 1.7 68 ◯ ◯ 12 1.8 300 550 4.0 6.076 ◯ ◯

TABLE 3 Flow rate of Ar gas for Oxygen efficiency Immersion depth, mmagitation, Nl/min/t in decarburization Run W_(CaO)/ Temp. eleva-Decarburiza- Temp. eleva- Decarburiza- in decarburization Sam- Evalu-No. W_(A1) tion period tion period tion period tion period period, %pling ation Comp. 13 0.6 250 600 3.9 2.6 48 X X 14 4.5 300 600 4.1 2.943 Δ X 15 1.9  50 600 3.8 3.2 44 X X 16 1.0 450 600 4.2 3.5 42 ◯ X 172.1 300 400 4.0 2.7 49 X X 18 1.5 300 800 3.9 3.0 43 ◯ X 19 1.3 300 6002.5 2.7 45 ◯ X 20 2.1 350 650 5.6 3.3 48 X X 21 1.6 300 650 3.5 1.2 34 ◯X 22 1.8 300 600 4.0 8.5 49 X X

Example 3

The effect of addition of CaO and the influence of the slag thicknesswere examined by adding CaO in the oxygen blowing decarburizationrefining period to the vacuum tank under the following experimentalconditions.

Runs of Example 3 were carried out in a 150-t molten steel ladle using amolten 16% Cr stainless steel, which had been roughly decarburized to [%C]=0.7% in a converter. For the runs, oxygen blowing decarburization wascarried out at an oxygen blowing rate of 24.0 Nm³/h/t until [% C]reached 0.05%. Further, for all the runs, the flow rate of an argon gasfor agitation in the oxygen blowing decarburization period was 3.3Nl/min/t.

Experimental results show that, when the experimental conditions fellwithin the scope of the present invention, as shown in Table 4, oxygenblowing decarburization of a molten steel in vacuo could be carried outwhile maintaining high productivity without deterioration in operationderived from splashing.

TABLE 4 Thickness Oxygen Melt loss of Run of slag in Composition of slagSplash- efficiency in refracto- Evalu- No. tank, mm (% Cao/% SiO₂) (%Al₂O₃) (% Cr₂O₃) ing decarburization, % ries ation Inv. 1 350 2.5 21 28Slight 76 Slight ◯ 2 600 2.3 25 35 Slight 74 Slight ◯ 3 100 3.1 16 26Slight 70 Slight ◯ 4 1000  2.7 18 29 Slight 71 Slight ◯ 5 250 2.1 15 31Slight 78 Slight ◯ 6 400 2.9 22 35 Slight 68 Slight ◯ 7 650 1.0 10 38Slight 75 Slight ◯ 8 500 4.0 23 24 Slight 72 Slight ◯ 9 350 3.4 5 26Slight 76 Slight ◯ 10 550 2.5 30 27 Slight 71 Slight ◯ 11 600 2.4 20 40Slight 74 Slight ◯ Comp. 12  70 3.1 15 31 Severe 72 Slight X 13 1200 2.5 18 24 Slight 34 Severe X 14 300 0.6 24 36 Slight 71 Severe X 15 2504.5 21 27 Severe 72 Slight X 16 600 2.7 3 29 Severe 74 Slight X 17 7502.4 38 24 Slight 70 Severe X 18 450 3.0 19 55 Severe 71 Slight X

TABLE 5 Run No. of Ex. 1 2 3 4 5 High carbon h/H 0.3 0.4 0.1 0.6 0.2concentra- Flow rate of 1.7 1.9 1.8 1.6 0.3 tion region inert gas*,Nl/min Low carbon Reduction rate of 6.7 7.1 5.2 2.6 3.1 concentra-oxygen gas flow tion region rate*, Nm³/hr/min Increase or de- Done DoneDone Done Done crease in snorkel depth h (i) Splashing ◯ ◯ ◯ ◯ ◯ (ii)Oxygen efficiency in decarburization, %: High carbon conc. region 74 7171 70 75 Low carbon conc. region 72 71 70 69 70 (iii) Fixation betweenvacuum None None None None None tank and ladle (iv) Productivity ◯ ◯ ◯ ◯◯ Chromium loss Overall evaluation of (i) to ◯ ◯ ◯ ◯ ◯ (iv) *Amount perton of molten steel to be treated

TABLE 6 Run No. of Ex. 6 7 8 9 High carbon h/H 0.3 0.2 0.2 0.6concentra- Flow rate of 4.0 1.9 2.3 2.1 tion region inert gas*, Nl/minLow carbon Reduction rate of 5.6 0.6 12.5 6.1 concentra- oxygen gas flowtion region rate*, Nm³/hr/min Increase or de- Done Done Done Done creasein snorkel depth h (i) Splashing ◯ ◯ ◯ ◯ (ii) Oxygen efficiency indecarburization, %: High carbon conc. region 71 72 71 77 Low carbonconc. region 72 68 76 71 (iii) Fixation between vacuum None None NoneNone tank and ladle (iv) Productivity ◯ ◯ ◯ ◯ Chromium loss Overallevaluation of (i) to ◯ ◯ ◯ ◯ (iv) *Amount per ton of molten steel to betreated

TABLE 7 Comp. run No. 1 2 3 4 5 High carbon h/H 0.06 0.8 0.2 0.3 0.3concentra- Flow rate of 1.9 1.8 0.15 5.5 2.2 tion region inert gas*,Nl/min Low carbon Reduction rate of 6.6 5.9 5.7 6.3 0.2 concentra-oxygen gas flow tion region rate*, Nm³/hr/min Increase or de- Done DoneDone Done Done crease in snorkel depth h (i) Splashing ◯ ◯ ◯ ◯ ◯ (ii)Oxygen efficiency in decarburization, %: High carbon conc. region 43 4538 42 73 Low carbon conc. region 71 70 33 69 31 (iii) Fixation betweenvacuum None None None None None tank and ladle (iv) Productivity ◯ ◯ ◯ ◯◯ Chromium loss Overall evaluation of (i) to X X X X X (iv) *Amount perton of molten steel to be treated

TABLE 8 Comp. run No. 6 7 High carbon h/H 0.2 0.2 concentra- Flow rateof 1.4 2.0 tion region inert gas*, N1/min Low carbon Reduction rate of16.2 6.6 concentra- oxygen gas flow tion region rate*, Nm³hr/minIncrease or de- Done Done crease in snorkel depth h (i) Splashing O O(ii) Oxygen efficiency in decarburization, %: HIgh carbon conc. region70 71 Low carbon conc. region 78 72 (iii) Fixation between vacuum NoneNone tank and ladle (iv) Productivity X O Chromium loss Overallevaluation of (i) to X X (iv) *: Amount per ton of molten steel to betreated

Example 4

An detailed experiment on decarburization refining in a high carbonconcentration region and a low carbon concentration region was carriedout in the same manner as in Example 1.

Experimental results are summarized in Tables 5 10 to 8.

FIGS. 15 to 17 are graphs respectively showing the relationship betweenthe oxygen efficiency in decarburization and the immersion ratio (h/H),the relationship between the oxygen efficiency in decarburization andthe flow rate (N) of an inert gas, and the relationship between thereduction rate (R) of the flow rate of an oxygen gas.

As shown in FIGS. 15 and 16, the oxygen efficiency in decarburizationcan be brought to not less than 65% by maintaining the immersion ratio(h/H) at 0.1 to 0.6 and maintaining the flow rate (N) of the inert gasat 0.3 to 4.0 Nl/min/t.

Further, as is apparent from FIG. 17, the oxygen efficiency indecarburization can be maintained at not less than 65% withoutdeteriorating the productivity by bringing the reduction rate (R) of theoxygen gas flow rate to the range of 0.6 to 12.5 Nm³/h/t/min. In FIG.17, the hatched portion is a region where the productivity isdeteriorated due to prolonged treatment time and the like in the wholerefining treatment.

For example, run No. 1 of Example 4 is a run where in the high carbonconcentration region, the oxygen gas flow rate was maintained in thespecified range, that is, at 3 to 25 Nm³/h/t, while, as specified inTable 5, maintaining the immersion ratio (h/H) and the inert gas flowrate (N) respectively at 0.3 and 1.7 Nl/min/t, and, in the subsequentlow carbon concentration region, the oxygen gas flow rate (Q) wasreduced at a rate of 6.7 Nm³/h/t/min and the immersion depth (h) of thesnorkel 14 was decreased and/or increased.

As is apparent from the results shown in the columns (i) to (iv) ofTables 5 and 6, for example, in run No. 1 of Example 4, the splashing(i) was small, that is, the prevention of splashing was good (O), andthe oxygen efficiency in decarburization (ii) in the high carbonconcentration region and the oxygen efficiency in decarburization (ii)in the low carbon concentration region were respectively 74% and 72%which were a higher level than a predetermined level (65%) required forproduction control.

Further, fixation between the vacuum tank and the ladle (iii) was notobserved, and the chromium loss (iv) was on a lower level than apredetermined level, that is, the prevention of the chromium loss wasgood (O).

Thus, run No. 1 of Example 4 satisfied all the requirements (i) to (iv),and the overall evaluation was good (O).

As is apparent from the results, in all of runs No. 1 to No. 9 ofExample 4, good overall evaluation (O) could be obtained by properlyregulating and maintaining various conditions for the decarburizationrefining.

On the other hand, Tables 7 and 8 show comparative runs No. 1 to No. 8where the conditions were outside the scope of the present invention.For all of runs No. 1 to No. 8, the overall evaluation was poor (X).

Run No. 1 is a comparative run wherein the immersion ratio (h/H) was setat 0.06 which was a value outside the range (0.1 to 0.6) specified inthe present invention. In this case, the oxygen efficiency indecarburization in the high carbon concentration region was 43%, i.e., alower value than the reference value 65% for the evaluation.

Run No. 2 is a comparative run wherein the oxygen gas flow rate (Q) wasset at a value which was outside and higher than the range (3 to 25Nm³/h/t) specified in the present invention. In this run, the oxygenefficiency in decarburization in the high carbon concentration regionwas as low as 45%.

Run No. 3 is a comparative run wherein the inert gas flow rate (N) wasset at 0.15 Nl/min/t, i.e., a value outside the range (0.3 to 4.0Nl/min/t) specified in the present invention. In this run, the oxygenefficiency in decarburization in the high carbon concentration regionwas 38%, a lower value than that in run No. 2.

Run No. 4 is a comparative run wherein the oxygen gas flow rate in thehigh carbon concentration region was set at a value which was outsideand lower than the range (3 to 25 Nm³/h/t) specified in the presentinvention. In this run, the oxygen efficiency in decarburization in thehigh carbon concentration region was 42%, i.e., poor.

Run No. 5 is a comparative run wherein the reduction rate (R) of theoxygen gas flow rate in the low carbon concentration region was set at0.2 Nm³/h/t/min, a value outside the range (0.5 to 12.5 Nm³/h/t/min)specified in the present invention. In this run, the oxygen efficiencyin decarburization in the low carbon concentration region was as low as31%.

Run No. 6 is a comparative run wherein the reduction rate (R) of theoxygen gas flow rate in the low carbon concentration region was set at16.2 Nm³/h/t/min, a value exceeding the range (0.5 to 12.5 Nm³/h/t/min)specified in the present invention. In this run, the amount of chromiumloss or the like became large and not negligible, resulting inremarkably lowered productivity.

Run No. 7 is a last comparative run wherein the decarburization refiningwas carried out with the immersion depth (h) of the snorkel 14 submergedin the vacuum tank in the low carbon concentration region being fixed.In this run, slag 12 was deposited on the molten steel surface of theinner wall of the ladle 13 and the outer wall of the snorkel 14, causingfixation between the ladle and the snorkel, which was an obstacle to theproduction.

Example 5

An experiment on degassing was carried out using a vacuum refiningapparatus on a scale of 150 tons (t). Table 9 shows runs according tothe present invention, and Table 10 shows comparative runs.

In any of run Nos. 1 to 14 according to the present invention shown inTable 9 and run Nos. 15 and 25 (comparative runs) shown in Table 10,after a molten crude stainless steel having a chromium concentration ofnot less than 5% (mainly 10 to 20%) was roughly decarburized to a carbonconcentration of about 0.7% in a converter, the molten steel wassubjected to oxygen blowing decarburization refining in vacuo followedby degassing for 30 to 60 min. The target carbon concentration of thesteel species in all runs according to the present invention is not morethan 0.002% (20 ppm). The oxygen gas blowing rate during the oxygenblowing decarburization refining was kept constant, i.e., at 20 Nm³/h/t.

Run No. 15 is a comparative run wherein [% C] during a stop of oxygenblowing was 0.012% (lower than 0.02%). This resulted in increasedoxidation of chromium during oxygen blowing. Run No. 16 is a comparativerun wherein [% C] during a stop in oxygen blowing was 0.125% (largerthan 0.1%). This resulted in increased attained carbon concentration,making it impossible to produce desired stainless steel within apredetermined treatment time range. Run No. 17 is a comparative runwherein the degree of vacuum during a top of oxygen blowing was higherthan the degree of vacuum specified in the present invention. In thisrun, due to an insufficient amount of oxygen during degassing, thedecarburization could not be smoothly carried out. Run No. 18 is acomparative run wherein the degree of vacuum during a stop of oxygenblowing was lower than the degree of vacuum specified in the presentinvention. In this run, the oxidation of chromium was unfavorablyincreased.

Run No. 19 is a comparative run wherein the attained degree of vacuum atthe time of degassing was 12 Torr. In this run, the attained [% C] washigh due to high equilibrium attained value. Run No. 20 is a comparativerun wherein the amount of oxygen reblown at the time of degassing wassmall. In this run, the amount of oxygen in the molten steel duringdegassing was so low that the decarburization could not smoothlyproceed, resulting in high attained [% C]. Run No. 21 is a comparativerun wherein the amount of oxygen reblown was large. In this run,chromium was oxidized due to the presence of excessive oxygen.

Run No. 22 is a comparative run wherein the degree of vacuum duringreblowing of oxygen was higher than the range specified in the presentinvention. In this run, the amount of oxygen to be dissolved in themolten steel was insufficient. This caused a lowered decarburizationrate, resulting in high attained [% C]. Run No. 23 is a comparative runwherein the degree of vacuum during reblowing of oxygen was lower thanthe range specified in the present invention. In this run, the oxidationof chromium proceeded. Run No. 24 is a comparative run wherein theamount of an argon gas, which is one example of the gas for agitation,was smaller than that specified in the present invention. In this run,since the agitation of the molten steel was unsatisfactory, the attained[% C] was high. Run No. 25 is a comparative run wherein the amount ofthe argon gas for agitation was larger than the range specified in thepresent invention. In this run, the attack of the refractory by the gaswas severe, resulting in increased damage to the refractory. Run No. 26is a comparative run wherein the amount of the residual slag wasincreased. In this run, since the free surface, which is a main site forthe decarburization reaction, was not satisfactorily ensured, thedecarburization rate was so low that the attained [% C] was large.

TABLE 9 Degree of Degree Amount Amount [c] vacuum of Flow rate of ofduring during Attained Amount vacuum of Ar gas residual Decarbu-chromium stop of stop of degree of during for slag rization Damageoxidized oxygen oxygen of oxygen re- agita- within rate Attain- toduring Run blowing, blowing, vacuum, reblown, blowing, tion, tank,constant, ed [C], refrac- oxygen Evalu- No. % Torr Torr Nm³/t TorrNl/min/t t/m³ l/min ppm tory blowing ation Inv. 1 0.025 50 1.5 1.9 155.5 0.35 0.19 7 Small Small ⊚ 2 0.034 65 2.0 2.5 23 6.1 0.42 0.17 9Small Small ⊚ 3 0.01 45 2.5 1.5 27 6.3 0.28 0.11 9 Small Small ⊚ 4 0.1075 1.0 2.3 18 4.8 0.35 0.14 11 Small Small ⊚ 5 0.041 10 2.3 1.8 8 5.20.44 0.15 12 Small Small ⊚ 6 0.029 100 0.9 2.8 25 6.6 0.38 0.12 8 SmallSmall ⊚ 7 0.031 35 5.0 3.3 22 5.9 0.41 0.13 11 Small Small ⊚ 8 0.043 601.1 0.3 19 3.9 0.45 0.11 9 Small Small ⊚ 9 0.051 65 3.4 5.0 26 6.8 0.220.13 12 Small Small ⊚ 10 0.032 45 2.9 2.1 5 5.2 0.19 0.15 11 Small Small⊚ 11 0.036 40 1.6 3.9 30 4.9 0.25 0.14 13 Small Small ⊚ 12 0.024 25 0.81.7 17 2.5 0.36 0.11 8 Small Small ⊚ 13 0.037 15 1.4 4.1 20 8.5 0.280.12 10 Small Small ⊚ 14 0.028 20 2.1 2.4 9 5.0 1.2 0.12 11 Small Small⊚

TABLE 10 Degree of Degree Amount Amount [c] vacuum of Flow rate of ofduring during Attained Amount vacuum of Ar gas residual Decarbu-chromium stop of stop of degree of during for slag rization Damageoxidized oxygen oxygen of oxygen re- agita- within rate Attain- toduring Run blowing, blowing, vacuum, reblown, blowing, tion, tank,constant, ed [C], refrac- oxygen Evalu- No. [%] Torr Torr Nm³/t TorrNl/min/t t/m³ l/min ppm tory blowing ation Comp. 15 0.012 15 3.5 2.2 156.3 0.36 0.10 17 Small Large X 16 0.125 75 2.6 1.7 21 5.9 0.24 0.06 89Small Small X 17 0.031 7 0.6 2.9 10 4.5 0.19 0.03 96 Small Small X 180.039 125 3.2 1.3 18 3.9 0.45 0.12 15 Small Large X 19 0.041 25 12 3.621 4.6 0.23 0.07 104 Small Small X 20 0.036 30 2.2 0.2 20 6.4 0.35 0.0583 Small Small X 21 0.045 25 2.6 6.7 16 6.6 0.38 0.13 13 Small Large X22 0.052 45 3.3 3.4 3.5 7.3 0.24 0.04 79 Small Small X 23 0.027 20 3.52.6 50 7.5 0.22 0.11 17 Small Large X 24 0.036 20 1.6 1.6 13 1.8 0.310.03 87 Small Small X 25 0.026 25 2.7 2.3 19 12.5 0.44 0.14 11 LargeSmall X 26 0.043 35 3.9 1.9 23 6.6 1.45 0.04 74 Small Small X

Example 6

This example was carried out using a vacuum degassing apparatus on ascale of 175 tons. After a molten steel having [% C] about 0.7% and [%Cr] not less than 5% (mainly 10 to 20%) was produced by the melt processin a converter, the molten steel was then subjected to oxygen blowingdecarburization refining to [% C]=0.01% in a vacuum refining apparatushaving a construction shown in FIG. 1. After the oxygen blowing wasstopped, the molten steel was degassed for 30 min by mere agitationthrough blowing of an inert gas from the bottom of the ladle, therebybringing the C concentration to not more than 20 ppm.

Table 11 shows runs in the degassing period according to the presentinvention in comparison with comparative runs. Run No. 5 is acomparative run wherein the K value exceeded 3.5. In this run, the areaof the gas bubble activated surface and the agitation intensity weresatisfactorily maintained, and the attained [C] was low. However, theerosion of the refractory was accelerated due to increased amount of thegas blown and the like. Therefore, conditions in run No. 5 areunsuitable for practical use.

As is apparent from Table 11, according to the present invention, areduction in loss of chromium by oxidation by utilizing the effectattained by properly regulating the oxygen feed rate and properlyregulating the state of agitation in the molten steel within the snorkelin the oxygen blowing period and, in addition, maintaining the gasbubble activated area and the surface agitation intensity in thedegassing period advantageously enables a high-purity stainless steel tobe efficiently produced by the melt process.

TABLE 11 Proportion of activated Carbon conc. Carbon conc. Run surfacebased on total before after Damage to Evalu- No. K-value molten steelsurface area, % treatment, ppm treatment, ppm refractory ation Inv. 12.4 85 100 8 ◯ ⊚ 2 0.5 80 102 10 ◯ ⊚ 3 3.5 85 104 6 ◯ ⊚ 4 3.1 10 105 12◯ ⊚ Comp. 5 4.5 85 111 7 X X 6 0.2 75 101 40 ◯ X 7 2.7  7 106 37 ◯ X VOD8 — — 104 45 Δ X

Example 7

An experiment was carried out, as follows, wherein aluminum forreduction was added after vacuum refining and degassing according to thepresent invention.

The experiment in this example was carried out using a vacuum refiningapparatus on a scale of 150 tons. A molten crude stainless steelcontaining a chromium concentration of not less than 5% (mainly 10 to20%) tapped from a converter was subjected to oxygen blowingdecarburization refining in vacuum and then degassed, followed byaddition of aluminum from the top of the vacuum tank to reduce Cr₂O₃produced during oxygen blowing, thereby recovering Cr. For all runs, thereduction time was 5 min.

Table 12 shows runs according to the present invention in comparisonwith comparative runs.

Runs No. 1 to No. 9 are runs according to the present invention. Run No.10 is a comparative run wherein the argon gas flow rate for agitation atthe time of the introduction of aluminum for reduction was less than 0.1Nl/min/t. In this run, the molten steel penetrated the porous plug,adversely influencing subsequent reduction. Run No. 11 is a comparativerun wherein the argon gas flow rate at the time of the introduction ofaluminum was excessive. In this run, bumping occurred immediately afterthe introduction of aluminum. Run No. 12 is a comparative run whereinthe degree of vacuum during the reduction was higher than 400 Torr. Inthis run as well, bumping occurred. Run Nos. 13 and 14 are comparativeruns wherein the flow rate of the argon gas for agitation after theintroduction of aluminum was less than 5 Nl/min/t or exceeded 10Nl/min/t. In this case, when the argon gas flow rate was less than 5Nl/min/t, the recovery of Cr₂O₃ was lowered. On the other hand, when theargon gas flow rate exceeded 10 Nl/min/t, a large pick-up of nitrogenwas observed. Run No. 15 is a comparative run wherein, when thedeposition and solidification of Cr₂O₃-containing slag on the upper partof the wall of the ladle was observed, aluminum was introduced with thevacuum tank submerged in the molten steel. In this case, the recovery ofCr₂O₃ was remarkably lowered.

TABLE 12 Degree of Ar flow vacuum Deposition and rate during duringsoldification introduc- introduc- Ar flow State of of Cr₂O₃- tion of Altion of rate after vacuum tank containing for aluminum for introduc-during slag on upper Pick-up Run reduction, reduction, tion of Al, Bump-introduction part of ladle of [N], Recovery of Evalu- No. Nl/min/t TorrNl/min/t ing of aluminum wall ppm Cr₂O₃, % ation Inv. 1 0.3 450 8.0 NoneSubmerged in None 3 97 ◯ molten steel 2 0.5 600 5.7 None Submerged inNone 2 96 ◯ molten steel 3 0.1 550 7.5 None Submerged in None 2 96 ◯molten steel 4 3.0 630 8.2 None Submerged in None 3 97 ◯ molten steel 50.8 760 7.6 None Submerged in None 4 95 ◯ molten steel 6 2.4 400 7.5None Submerged in None 1 97 ◯ molten steel 7 1.3 500 5.0 None Submergedin None 2 95 ◯ molten steei 8 0.9 650 10.0  None Submerged in None 3 98◯ molten steel 9 1.7 760 8.3 None Lifted Fixed 4 96 ◯ Comp. 10 0.05 560Ar did not None Submerged in None 1 34 X flow* molten steel 11 4.2 4508.5 Bumped Submerged in None 5 65 X molten steel 12 0.8 200 7.4 BumpedSubmerged in None 1 63 X molten steel 13 0.4 480 3.5 None Submerged inNone 3 73 X molten steel 14 0.6 550 12.9  None Submerged in None 15 98 Xmolten steel 15 0.3 760 7.8 None Submerged in Fixed 2 65 X molten steel*Ar did not flow due to a trouble of penetration of the molten steelinto the porous plug.

Example 8

The protection of a snorkel in a vacuum tank for vacuum refining of amolten stainless steel according to the present invention was carriedout as follows.

At the outset, a molten steel, having a weight of 150 tons (t),comprising 13% by weight of chromium, 0.7% by weight of carbon, and 0.03to 0.20% by weight of silicon was prepared by the melt process in aconverter, and the molten steel was poured into a ladle 13.

In pouring the molten steel, the amount of slag poured from theconverter was regulated to about 1000 kg (containing 30% by weight ofSiO₂), and, in the vacuum refining apparatus 10 shown in FIG. 1,decarburization refining, degassing refining, and reduction refiningwere further carried out.

Further, in order to regulate the slag and the acceleration of reductionrefining, CaO and metallic aluminum were added in such a manner that CaOwas dividedly added in two or three portions in the degassing refiningand the metallic aluminum was dividedly added in two or three portionsat the time of the initiation of the reduction of the reduction refiningand in the course of the reduction refining.

In this case, for slags No. 1 to No. 4 according to the presentinvention shown in Table 13, CaO was regulated to 8 to 18 kg/t, and themetallic aluminum was regulated to 6 to 18 kg/t in terms of Al₂O₃. Inparticular, in slag No. 4, the amount of slag poured from the converterwas about 1.5 times, resulting in increased SiO₂ content derived fromthe slag composition.

Next, the slag regulated to the composition shown in Table 13 was coatedonto the snorkel 14 in its portion from the lower end thereof to 500 mmfrom the lower end to form a 30 mm-thick coating by single immersion.Further, the coating, standing-by, and refining under reduced pressurewere repeated. The results were compared with the conventional techniquewhere there was no slag coating.

Regarding the number of times of use of the snorkel, as compared withthe conventional technique where vacuum refining is repeatedly carriedout under reduced pressure with no coating being provided, the presentinvention could increase the number of times of use of the snorkel by1.5 times by virtue of a reduction in melt loss caused by the moltensteel or slag and a reduction in spalling due to heat load.

By virtue of the increase in number of times of use of the snorkel, therefractory cost of the snorkel of the present invention, when therefractory cost of the conventional technique was presumed to be 1, wasabout 0.6, indicating that a marked reduction in cost of 40% could beachieved.

Further, since the slag for coating utilizes additives and the producedcomposition, which can effectively function also in decarburizationrefining and degassing refining in the refining apparatus under reducedpressure, particularly the acceleration of the reduction refiningreaction, both the protection of the refractory constituting the snorkeland the acceleration of the refining can be synergistically utilized,simultaneously improving the refining efficiency, the service life ofthe snorkel, the reduction in refractory cost and the like.

Substantially the same effect could be attained when coating was carriedout a plurality of times by repeating the immersion and standing to forma 60 mm-thick coating. Coating by a plurality of times permitted theloss attributable to spalling created by the high-temperature moltensteel and the heat of slag to be prevented in reuse of the snorkel,offering better results.

TABLE 13 No. 1 2 3 4 CaO, wt % 50.0 37.0 22.0 48.0 SiO₂, wt % 7.0 10.017.0 25.0 Al₂O₃, wt % 35.0 41.0 48.0 17.0 Cr₂O₃, wt % 2.0 5.0 6.0 4.0MgO 5.5 6.0 6.0 5.0 Total of FeO 0.5 1.0 1.0 1.0 and Fe₂O₃, wt % Totalof Al₂O₃ 85.0 78.0 70.0 65.0 and CaO, wt % Al₂O₃/CaO 0.70 1.11 2.18 0.35

Example 9

The following experiment was carried out using a vacuum refiningapparatus shown in FIG. 24 according to the present invention.

Tables 14 and 15 show the results of vacuum decarburization refining forrun Nos. 1 to 6 according to the present invention wherein vacuumdecarburization refining conditions, such as the inner diameter D_(L)and the inner sectional area S_(L) (m²) of a larger-diameter section 36corresponding to a freeboard section, the length A of thelarger-diameter section, the oxygen gas blowing distance L, and theinner sectional area S_(s) (m²) of a smaller-diameter section 37 havingan inner diameter D_(s), were set at respective various values.

As is apparent from Tables 14 and 15, in run Nos. 1 to 6 according tothe present invention wherein the (D_(L)/L) ratio and the (S_(s)/S_(L))ratio, which specify the geometrical configuration of the vacuum tank 15in the vacuum refining, were set respectively at 0.5 to 1.2 and 0.5 to0.9, the deposition of the metal within the vacuum tank and the meltloss of the refractory corresponding to the horizontal position of theportion just above the molten steel surface (the portion just above thehot spot) were very small (or did not occur), and it is apparent that,as indicated by mark O in the table, the refractory cost was maintainedwithin a predetermined level range and the overall evaluation wasregarded as good (O).

The term “oxygen efficiency in decarburization” refers to the proportionof the amount of the oxygen gas contributed to the decarburizationreaction relative to the total amount of the oxygen gas fed through theoxygen lance. For runs No. 1 to No. 6 according to the presentinvention, the oxygen efficiency in decarburization was on a level of 68to 78%.

The intimately mixing time is an index of the degree of agitation of themolten steel 11 in the vacuum refining and, for example, is expressed inthe time taken from the introduction of a metallic element or the likeas a label in the molten steel to the point of time when theconcentration of the metallic element become even or constant. For runsNo. 1 to No. 6 according to the present invention, the intimately mixingtime was in the range of from 38 to 51 sec.

Incidentally, in Table 16, runs No. 1 to No. 4 are comparative runswherein any one of the (D_(L)/L) ratio and the (S_(S)/S_(L)) ratio wasoutside the proper range.

Run No. 1 is a comparative run wherein the (D_(L)/L) ratio was 0.4 andoutside the proper range. In this run, the melt loss of the refractorycorresponding to the horizontal position of the portion immediatelyabove the molten steel surface was significant. As a result, run No. 1was evaluated as unacceptable (X).

Run No. 2 is a comparative run wherein the (D_(L)/L) ratio was 1.5, thatis, significantly outside the proper range. In this run, the force bywhich oxygen was blown against the molten steel surface was so weak thatthe decarburization reaction efficiency was remarkably lowered. As aresult, run No. 2 was evaluated as unacceptable (X).

Run No. 3 is a comparative run wherein the (S_(S)/S_(L)) ratio was 0.4,that is, lower than the proper range. In this run, the flow resistanceof the exhaust gas was so large that the degree of vacuum was lowered.As a result, run No. 3 was evaluated as unacceptable (X).

Run No. 4 is a comparative run wherein the (S_(S)/S_(L)) ratio was 1.0,that is, larger than the proper range. In this run, the deposition ofthe metal within the vacuum tank was significant. As a result, run No. 4was evaluated as unacceptable (X).

TABLE 14 Run No. of inv. 1 2 3 4 Conditions for Larger- Length A 23002300 2300 2300 vacuum diameter Inner diameter D_(L) 2100 2100 2100 2100decarburization section Inner sectional area S_(L) 3.46 3.46 3.46 3.46refining Oxygen gas Blowing distance L 2625 2334 2334 3000 Innersectional area of Smaller-diameter section S_(s) 2.76 2.42 1.86 2.76Unit of area: D_(L)/L 0.8 0.9 0.9 0.7 m² S_(s)/S_(L) 0.8 0.7 0.54 0.8Fan-shaped shields Number of shields disposed 0 0 0 0 Interval, mm — — —— Results of Deposition of metal within vacuum tank None None None Nonevacuum Melt loss of refractory on portion decarburization immediatelyabove molten steel surface None None None None refining Oxygenefficiency in decarburization, % 75 78 68 75 Intimately mixing time 45sec 43 sec 51 sec 38 sec Refractory cost ◯ ◯ ◯ ◯ Overall evaluation ◯ ◯◯ ◯

TABLE 15 Run No. of inv. 5 6 7 Conditions for Larger- Length A 2300 23002300 vacuum diameter Inner diameter D_(L) 2100 2100 2100 decarburizationsection Inner sectional area S_(L) 3.46 3.46 3.46 refining Oxygen gasBlowing distance L 4200 1750 2330 Inner sectional area ofSmaller-diameter section S_(s) 3.11 2.76 3.46 Unit of area: D_(L)/L 0.51.2 0.9 m² S_(s)/S_(L) 0.9 0.8 1.0 Fan-shaped shields Number of shieldsdisposed 0 0 3 Interval, mm — — 150 Results of Deposition of metalwithin vacuum tank None None None vacuum Melt loss of refractory onportion decarburization immediately above molten steel surface None NoneNone refining Oxygen efficiency in decarburization, % 74 73 76Intimately mixing time 42 sec 46 sec 46 sec Refractory cost ◯ ◯ ◯Overall evaluation ◯ ◯ ◯

TABLE 16 Comp. run No. 1 2 3 4 Conditions for Larger- Length A 2300 23002300 2300 vacuum diameter Inner diameter D_(L) 2100 2100 2100 2100decarburization section Inner sectional area S_(L) 3.46 3.46 3.46 3.46refining Oxygen gas Blowing distance L 5250 1400 3500 2625 Innersectional area of smaller-diameter section S_(s) 2.76 2.76 1.38 3.46Unit of area: D_(L)/L 0.4 1.5 0.6 0.8 m² S_(s)/S_(L) 0.8 0.8 0.4 1.0Fan-shaped shields Number of shields disposed 0 0 0 0 Interval, mm — — —— Results of Deposition of metal within vacuum tank None None NoneSevere vacuum Melt loss of refractory on portion decarburizationimmediately above molten steel surface Severe None None None refiningOxygen efficiency in decarburization, % 72 70 38 75 Intimately mixingtime 72 sec 70 sec 38 sec 75 sec Refractory cost X X ◯ ◯ Overallevaluation X X X X

Example 10

An experiment on burner blowing at the time of oxygen blowing accordingto the present invention was carried out as follows.

Runs No. 1 to No. 7 according to the present invention are runs whereinvacuum refining was carried out under down-blown oxygen decarburizationrefining conditions in vacuo as specified in Tables 17 and 18. Theresults (deposition of metal, state of damage to refractory, andevaluation) are summarized in these tables.

In the tables, the surface temperature in the canopy is the averagetemperature (° C.) in each period, and, in the column of the burnerblowing gas during oxygen blowing, the type of gas fed into burners 44-1and 44-2 shown in FIGS. 24 and 30 is indicated.

For example, run No. 1 is a run according to the present inventionwherein oxygen blowing decarburization refining was carried out in vacuoin such a manner that the front end distance L of the burner and theburner ejection angle θh were set respectively at 2.3 m and 50°, and theaverage surface temperature in the canopy in the oxygen blowing refiningperiod, the average surface temperature in the canopy in the non-oxygenblowing refining period, and the average surface temperature in thecanopy in the standing period were regulated respectively at 1520° C.,1500° C., and 800° C. by means of the burners 44-1 and 44-2.

In run No. 1 according to the present invention, there was no depositionof the metal in the canopy 35, and the loss of the refractory was verysmall. As a result, run No. 1 was evaluated good (O).

In runs No. 1 to No. 7 according to the present invention, maintainingthe surface temperature of the canopy during oxygen blowing (in theoxygen blowing refining period) and during non-oxygen blowing (in thenon-oxygen blowing refining period) in a predetermined range of 1200 to1700° C. by means of burners 16 and 17 resulted in prevention of thedeposition of the metal and minimized loss of the refractory, that is,provided good results (O).

Comparative runs No. 1 to No. 4 shown in Table 19 are comparative runswherein the surface temperature of the canopy in any one of the oxygenblowing period (oxygen blowing refining period) and the non-oxygenblowing period (non-oxygen blowing refining period) was outside thepredetermined range of from 1200 to 1700° C. For all of comparative runsNo. 1 to No. 4, the deposition of the metal or the loss of therefractory was significant. As a result, these comparative runs wereevaluated as unacceptable (X).

For example, comparative run No. 1 is a comparative run wherein oxygenblowing decarburization refining was carried out in vacuo in such amanner that the front end distance L of the burner and the burnerejection angle θh were set respectively at 3.5 m and 65°, and theaverage surface temperature in the canopy in the oxygen blowing refiningperiod, the average surface temperature in the canopy in the non-oxygenblowing refining period, and the average surface temperature in thecanopy in the standing period were regulated respectively at 1150° C.,1100° C., and 800° C.

In this case, as is apparent from Table 19, the front end distance ofthe burner was large, and the position of the front end was so low thatthe temperature of the canopy 35 was below the predetermined range,resulting in increased amount of deposition of the metal in the canopy35.

TABLE 17 Run No. of inv. 1 2 3 4 Conditions Surface temp. in canopyduring oxygen 1520 1560 1610 1520 for oxygen blowing, ° C. blowingSurface temp. in canopy during non-oxygen 1500 1480 1470 1500decarburiza- blowing, ° C. tion Surface temp. in canopy during standing,800 1200 1200 1200 refining in ° C. vacuo Front end distance of burnerL, m 2.3 1.8 2.1 1.5 Burner ejection angle θh, ° 50 55 45 47 Burnerblowing gas during oxygen blowing Oxygen gas + Oxygen gas + Oxygen gas +Oxygen gas + LPG LPG LPG LPG Results Deposition of metal within vacuumtank None None None None Loss of refractory Very small Very small Verysmall Very small Evaluation ◯ ◯ ◯ ◯

TABLE 18 Run No. of inv. 5 6 7 Conditions Surface temp. in canopy duringoxygen 1520 1700 1530 for oxygen blowing, ° C. blowing Surface temp. incanopy during non-oxygen 1500 1200 1300 decarburiza- blowing, ° C. tionSurface temp. in canopy during standing, 1200 800 1200 refining in ° C.vacuo- Front end distance of burner L, m 2.5 0.3 3.0 Burner ejectionangle θh, ° 47 20 90 Burner blowing gas during oxygen blowing Oxygengas + Oxygen gas + Oxygen gas + LPG LPG LPG Results Deposition of metalwithin vacuum tank None None None Loss of refractory Very small Verysmall Very small Evaluation ◯ ◯ ◯

TABLE 19 Comp. run No. 1 2 3 4 Conditions Surface temp. in canopy duringoxygen 1150 1760 1505 1625 for oxygen blowing, ° C. blowing Surfacetemp. in canopy during non-oxygen 1100 1495 1080 1810 decarburiza-blowing, ° C. tion Surface temp. in canopy during standing, 800 12001200 1200 refining in ° C. vacuo Front end distance of burner L, m 3.52.4 2.2 0.2 Burner ejection angle θh, ° 65 100 10 70 Burner blowing gasduring oxygen blowing Oxygen gas + Oxygen gas + Oxygen gas + Oxygengas + LPG LPG LPG LPG Results Deposition of metal within vacuum tankSevere None Severe None Loss of refractory Very small Severe Very smallSevere Evaluation X X X X

Example 11

An experiment on an evacuation duct shown in FIG. 32 was carried out asfollows.

Runs No. 1 to No. 4 according to the present invention shown in Table 20are runs wherein vacuum refining was carried out in such a manner thatoperation conditions, such as the inclination angle (θ₀) in anascendably inclined section 46 of an evacuation duct 16-1 and the actuallength (L₀) of the evacuation duct 16-1, were varied. The results of theoperation are summarized in Table 20.

For example, run No. 1 according to the present invention in Table 20 isa run wherein vacuum refining was carried out for about 5 days in such amanner that the inclination angle (θ₀) was brought to 45°, the actuallength (L₀) was brought to 22 m, and a dust pot 53 (metal pot) wasdisposed below a descendably inclined section 48.

As shown in the column of the results of operation, the state ofdeposition of dust in a duct inlet 45 was very small, there was nodamage to a gas cooler 55 caused by the deposition of dust, and theattained degree of vacuum could be maintained at 0.5 Torr. As a result,run No. 1 was evaluated as good (O).

As is apparent from the results of runs No. 2 to No. 4, good resultscould be obtained by bringing the inclination angle (θ₀) and the actuallength (L₀) to respective predetermined ranges and providing the metalpot 53.

Comparative runs No. 1 to No. 4 corresponding to the runs according tothe present invention are shown in Table 21.

For example, comparative runs No. 1 and No. 2 in Table 21 arecomparative runs wherein the inclination angle (θ₀) in the ascendablyinclined section 46 was set at 15° for comparative run No. 1 and 0° forcomparative run No. 2 which were outside the proper range of from 30 to60°. In these runs, the deposition of dust in the duct inlet 45 wassignificant, the pressure loss in the evacuation duct 16-1 wasincreased, and the attained degree of vacuum was on a level of 35 Torrand 45 Torr. As a result, comparative runs No. 1 and No. 2 wereevaluated as unacceptable (X).

Comparative run No. 3 is a comparative run wherein no metal pot wasprovided. In this run, the deposition of dust in the duct inlet 45 wasvery small. However, dust, which flowed in the duct beyond the top 47 ofthe ascendably inclined section 46, reached the gas cooler 55 withoutbeing collected. This caused remarkable damage to the gas cooler andresulted in a low attained degree of vacuum of 40 Torr.

Comparative run No. 4 is a comparative run wherein the actual length(L₀) of the evacuation duct 16-1 was 6 m, that is, outside the properrange (15 to 50 m). In this run, despite the provision of the metal pot53, since the actual length (L₀) was short, the amount of inflow of thedust in the gas cooler 55 was increased, resulting in increased damageto the gas cooler 55.

TABLE 20 Run No. of inv. 1 2 3 4 Operating Inclination angle inascendably inclined 45° 60° 30° 40° conditions section, θ₀ Actual lengthof evacuation duct, L₀ 22 m 25 m 20 m 15 m Metal pot Provided ProvidedProvided Provided Results of Deposition of metal in duct inlet Verysmall Very small Very small Very small Operation Damage to gas coolerNone None None None Attained degree of vacuum, Torr 0.5 0.8 0.9 1.0Evaluation ◯ ◯ ◯ ◯

TABLE 21 Comp. run No. 1 2 3 4 Operating Inclination angle in ascendably15° 0° 45° 50° conditions inclined section, θ₀ Actual length ofevacuation duct, L₀ 19 m 23 m 25 m 6 m Metal pot Provided Provided Notprovided Provided Results of Deposition of metal in duct inlet SevereSevere Very small Very small Operation Damage to gas cooler None NoneSevere Severe Attained degree of vacuum, Torr 35 45 40 45 Evaluation X XX X

INDUSTRIAL APPLICABILITY

According to the present invention, in straight-barrel type vacuumrefining, optimal regulation of the pressure within a vacuum tank in analuminum temperature elevation period and, in addition, feed of anoxygen gas at an optimal flow rate according to the carbon concentrationwhile regulating the slag component in the oxygen blowingdecarburization period can inhibit oxidation loss of chromium during thealuminum temperature elevation, can improve the oxygen efficiency indecarburization in the oxygen blowing decarburization period, and, inthe high carbon concentration region, can prevent splashing within asnorkel of the vacuum tank and the fixation of the submerged section ofthe nozzle by slag. Therefore, the method for refining of a molten steelaccording to the present invention is very advantageous from theviewpoint of industry.

What is claimed is:
 1. A method for vacuum decarburization refining of a molten steel comprising: providing molten steel having a carbon concentration of 1.0 to 0.01% by weight in a ladle; providing a vacuum tank having a one-legged, straight barrel snorkel as a lower portion of said vacuum tank; immersing said one-legged, straight barrel snorkel of said vacuum tank into said molten steel in said ladle; evacuating an interior of said vacuum tank resulting in molten steel ascending in an interior of said one-legged, straight barrel snorkel immersed in said molten steel and into said interior of said vacuum tank; providing a liftable top-blown lance in an insert hole in a canopy of said vacuum tank; blowing oxygen gas through said top-blown lance into said molten steel at a flow rate in a range of 3 to 25 Nm²/hr/ton-steel; injecting inert gas into said molten steel from a low position of said ladle at a flow rate in a range of from 0.3 to 10 Nl/min/ton-steel; regulating a degree of vacuum in said vacuum tank at a high carbon concentration region, said carbon concentration of said molten steel in said high carbon concentration region being not less than a critical carbon concentration, said critical carbon concentration being in a range of 0.3 to 0.1% by weight; said degree of vacuum at said high carbon concentration region being regulated to a value in a range of −35 to −20 in terms of G defined by the following equation (1): G=5.96×10⁻³ ×T×ln(P/Pco)  (1) wherein Pco=760 {10^((−13800/T+8.75))}×(% C)/(% Cr)  (2) P<760 wherein T represents molten steel temperature, K, and P represents the degree of vacuum in the vacuum tank, Torr; thereby conducting oxygen blowing decarburization refining, followed by degassing.
 2. The method according to claim 1, wherein the flow rate of the inert gas injected from the low position of the ladle is brought, in the high carbon concentration region above the critical carbon concentration, to a range of from 0.3 to 4 Nl/min/ton-steel and is brought, in a low carbon concentration region not above the critical carbon concentration, to a range of from more than 4 to 10 Nl/min/ton-steel.
 3. The method according to claim 1, wherein, in a period of temperature elevation due to an oxidation of aluminum in a step before the oxygen blowing decarburization refining, the temperature of the molten steel is elevated in such a manner that the molten steel is poured into the ladle, the snorkel in the vacuum tank is immersed in the molten steel and, in addition, the degree of vacuum, P, in the atmosphere within the vacuum tank is controlled so as to give a G value, determined by the equation (1), of not more than −20, aluminum is added to the molten steel within the vacuum tank with the controlled degree of vacuum, and the oxygen gas is blown through the top-blown lance into the vacuum tank to oxidize aluminum, thereby elevating the temperature of the molten steel.
 4. The method according to claim 1, wherein quick lime in an amount corresponding to 0.8 to 4.0 W_(Al) (kg), wherein W_(Al) represents the amount of aluminum added for the temperature elevation, is introduced into the tank from the temperature elevation period to the oxygen blowing decarburization period and, in addition, the depth of immersion of the snorkel into the molten steel during the temperature elevation period is in the range of from 200 to 400 mm.
 5. The method according to claim 1, wherein, in the oxygen blowing decarburization period, an inert gas is injected into the ladle from the low position of the ladle under conditions satisfying a requirement that a activated surface area is brought to not less than 10% of the total surface area of the molten steel and not less than 100% of a surface blown by an oxygen gas jet, thereby agitating the molten steel.
 6. The method according to claim 1, wherein, in the high carbon concentration region in the oxygen blowing decarburization period, quick lime and the like are introduced either at once or dividedly into the vacuum tank to form slag having a thickness of 100 to 1000 mm in terms of a still state, on the surface of the molten steel within the snorkel, which is then retained.
 7. The method according to claim 1, wherein, in the high carbon concentration region in the oxygen blowing decarburization period, the depth of immersion of the snorkel in the molten steel is in the range of from 500 to 700 mm.
 8. The method according to claim 1, wherein, in the low carbon concentration region in the oxygen blowing decarburization period, the oxygen blowing decarburization is carried out while decreasing the oxygen gas flow rate in a range of 0.5 to 12.5 Nm³/h/ton-steel/min and, at the same time, reducing the depth h of immersion of the snorkel in relationship with the depth H of the molten steel so as to satisfy the requirement h/H=0.1 to 0.6.
 9. The method according to claim 1, wherein, in the degassing period, the degassing treatment is carried out in such a manner that, during the stop of the blowing of oxygen through the top-blown lance, the degree of vacuum within the vacuum tank is brought to 10 to 100 Torr, and an inert gas is injected from the low portion of the ladle into the ladle while regulating the amount of the slag within the snorkel to not more than 1.2 ton/m² of the geometrical cross-sectional area of the snorkel and, at the same time, regulating the K value, determined by the following equation (3), to 0.5 to 3.5, thereby agitating the molten steel: K=log {S·H _(v) ·Q/P}  (3) wherein K: index of a agitation intensity at the activated surface; S: activated surface area, m²; H_(v): depth of injected inert gas, m; Q: flow rate of injected inert gas, Nl/min/ton-steel; and P: degree of vacuum within the tank, Torr.
 10. The method according to claim 1, wherein in reducing a metal oxide with aluminum after the completion of the degassing, in the aluminum reduction period, aluminum for reduction is added into the molten steel and, in the aluminum addition period, the flow rate of an inert gas, for agitation from the low portion of the ladle is brought to a range of from 0.1 to 3.0 Nl/min/ton-steel with the degree of vacuum within the tank being brought to not more than 400 Torr and, after the completion of the introduction of aluminum for reduction, the degree of vacuum within the tank is returned to the atmospheric pressure, followed by lifting of the vacuum tank and regulating the flow rate of the inert gas for agitation in a range from 5 to 10 Nl/min/ton-steel to reduce the metal oxide produced during the oxygen blowing, and permitting the recovery of a metal element.
 11. The method according to claim 1, wherein in reducing a metal oxide with aluminum after the completion of the degassing, in a period of the metal oxide reduction by aluminum, the pressure of the atmosphere within the vacuum tank is returned to the atmospheric pressure, the vacuum tank is lifted, and, at the same time, aluminum for reduction is added into the molten steel, and, in the aluminum addition period, the flow rate of an inert gas for agitation is brought in a range of from 0.1 to 3.0 Nl/min/ton-steel and, immediately after the completion of the addition of aluminum for reduction, the flow rate of the inert gas for agitation is brought in a range of 5 to 10 Nl/min/ton-steel to reduce the metal oxide produced during the oxygen blowing, and a metal element is recovered.
 12. The method according to claim 1, wherein, after the completion of the degassing or the reduction treatment with aluminum, the composition of slag after the completion of the refining is regulated so that the slag comprises by weight 55 to 90% in total of Al₂O₃ and CaO, not more than 10% of Cr₂O₃, and 7 to 25% of SiO₂ with the balance consisting of 2 to 10% in total of at least one member selected from FeO, Fe₂O₃, and MgO, the Al₂O₃/CaO ratio being in the range of from 0.25 to 3.0, followed by coating of the slag onto the surface of the snorkel of the refining apparatus after the decarburization refining.
 13. The method according to claim 1, wherein, during or after the completion of the oxygen blowing decarburization refining period, the vicinity of the canopy is heated, by means of a heating burner inserted into the vacuum tank, so that the surface temperature of the canopy in the vacuum tank is held at 1200 to 1700° C. 